CA1119176A - 3,5-dicarboxy dihydropyridine derivatives - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The invention is concerned with new 3,5-dicarboxy dihydropyridine derivatives having the general formula:

Description

-This invention relates to new additives for plastic materials and, more particularly, to new stabilizers for vinyl resins such as, for insta`nce, polyvinyl chloride and its copolymers.
The new additives for plastic materials with which the present invention is concerned are 3,5-dicarboxy - dihydropyridine derivatives having the general formula:
H R

R O-C ~ C-OR
~ ~ 2 Rl N l `

wherein the radicals Rl are the same and each represents a branched- or straight-chain alkyl group containing from 1 to 16 carbon atoms, a butenyl radical, a phenyl radical or a benzyl radical, the radicals R2 are the same end each represents a branched- or straight-chain alkyl group containing from l to 4 carbon atoms, an allyl radical, a propargyl radical, an ~-halogenoethyl group, a benzyl radical substituted on the ortho- or para-position by a halogen atom or a methyl radical, a cinnamoyl radical, a phenyl radical, a phenyl radical substituted on the para-position by a halogen atom or a methyl radical, a radical of the formula R4-O-(CH2)n-, wherein n is 1, 2, 3 or 4 and R4 represents a branched- or stralght-chain alkyl group containing from 1 to 4 carbon atoms, unsubstituted or substituted on the w-position by a halogen atom, or R4 represents a phenyl group, and R3 rcprcscnts a hydrogcn atom or a brancllcd- or strai~ht-chain alkyl group containing from 1 to 7 carbon atoms, with the proviso that when Rl represents a methyl radical and R3 a hydrogen atom, R2 does not represent an alkyl or a phenyl radlcal.

~31 - 1- ~

`: 111~17~i A particularly preferred class of compounds having the general formula I is represented by those compounds of the formula I in which R3 is hydrogen. These dihydro-pyridines bearing no substituents in position-4 show a greater stabilizing power than the corresponding 4-substituted dihydropyridines.
Some compounds of formula I are new. Among these new compounds, invention can be made of the following, preferred compounds:

2,6-Dimethyl-3,5-dicarboallyloxy-1,4-dihydropyridine (1) 2,6-Dimethyl-3,5-dicarbopropargyloxy-1,4-dihydropyridine (2) 2,6-Dimethyl-3,5-di-(2'-carbochloro-ethoxy)-1,4-dihydropyridine

(3) 2,6-Dimethyl-3,5-di-(4'-carbomethoxy-benzyloxy)-1,4-dihydro-pyridine(4) 2,6-Dimethyl-3,5-di-(2'-carbochloro-benzyloxy)-1,4-dihydro-pyridine (5) 2,6-Dimethyl-3,5-di-(2'-carbomethyl-benzyloxy)-1,4 dihydro-pyridine (6) 2,6-Dimethyl-3,5-di-(4'-carbochloro-benzyloxy)-1,4-dihydro-pyridine (7) 2,6-Dimethyl-3,5-dicarbomethoxy-4-hexyl-1,4-dihydropyrldine (8) 2,6-Dimethyl-3,5-dicarbomethoxy-4-propyl-1,4-dihydropyridine ..
(9) 2,4,6-Trimethyl-3,5-dicarbophenoxy-1,4-dihydropyridine (10) 2,4,6-Trimethyl-3,5-di-(4'-carbomethylphenoxy)-1,4-dihydro-pyridine (11) 2,4,6-Trimethyl-3,5-dicarbobenzyloxy-1,4-dihydropyridine (12) 2,4,6-Trimethyl-3,5-dicarboallyloxy-1,4-dihydropyridine (13) 2,4,6-Trimethyl-3,5-di-(4' carbochloro-benzyloxy)-1,4-dihydropyridine(14) ' ' ', ' ' , 111~176 2,6-Dimethyl-3,5-dicarbomethoxy-4-(1-propenyl)-1,4-dihyd~o-pyridine (15) 2,6-Dibenzyl-3,5-dicarbethoxy-1,4-dihydropyridine (16) 2,6-Di-(3-butenyl)-3,5-dicarbomethoxy-1,4-dihydropyridine (17) 2,6-Diphenyl-3,5-dicarbomethoxy-1,4-dihydropyridine (18) - 2 a -` ` ` 1~191~6 2,6-Dimethyl-3,5-di(2" -carbochloro-2' ethoxy-ethoxy)-1,4~ ~;
dihydropyridine (19) 2,6-Ditridecyl-3,5-dicarbomethoxy-1,4-dihydropyridine (20) Some other compounds of fo~mula I are already known.
These known compounds that are ~isted hereunder have never-the less to be considered as new stabilizers of polyvinyl chloride: . ;
2~6-dimethyl-3~5-dicarbobenzyloxy-1~4-dihydrOpyridine (21) ~' 2,6-dimethyl-3,5-dicarbocinnamyloxy-1,4-dihydropyridine (22) ,2,6-dimethyl-3,5-dicarbomethoxy-4-phenyl-1,4-dihydropyridine ~:
,~23) -:~
2,6-diphenyl-3,5-dicarboethoxy-1,4-dihydropyridine (24) ~,-2,'6-dimethyl-3,5-di-(2'-carbomethoxy-ethoxy)-1,4-dihydro- ~' -pyridine (25) 2,6-dimethyl-3,5-di-(2'carboethoxy-ethoxy)-1,4-dihydro- , pyridine (26) -:
2,6-dimethyl-3,5-di-(2'-carbobutoxy-ethoxy)~1,4~dihydro-pyridine (27) ' '~
2,6-dimethyl-3,5-di-(2'carbophenoxy-ethoxy)-1 ! 4-dihydro- '' pyridine (28), and ~ , 2,4,6-trimethyl-3,5-dicarbomethox,y-1,4-dihyd~opyridlne (29) The invention also rela,tes to the process of prepa, .
ratlon of the compounds of form~la I.
The compounds of the invention may be prepared according to the HANTZSCH synthesis, d,e,scribed in Chemical Revlews 72, I, 1972, by reacting a compound of the general ormula:

o Rl-C-CT~2-COOR2 II

- 30 wherein Rl and R2 have the same meanings as in formula I, with an aldehyde of the general formula~

:E3 '. ' ' ' . ; '-' ' " .: , ,-wherein R3 has the same meanings as in formula I and with ammonia.

- 3 a -:

~1191~6 The compounds of formula II, wherein Rl represents a methyl radical, can be prepared by one or the other of the following methods:
1) By reacting compound of the formula H2 C=C-0\ with CH2 -C=O
an alcohol of the general formula :

wherein R2 has the same meanings as in formula I, as described in Journal of Chemical Society, 854-60, 1954 and in Organic Synthesis 42, 28, 1962;
2) By transesterifying in a known manner methyl acetoacetate O O
Il . :
of the formula CH3-C-CH2-C-OCH3 by means of an alcohol of the above formula IV.
The compounds of general formula II, wherein Rl represents an alkyl chain containing from 2 to 16 carbon atoms may be prepared by reacting a halogenated derivative of the general formula :

wherein R5 has the same meanings as Rl as defined above but having one carbon atom less, with a compound of formula II
wherein Rl represents a methyl group, as described in Journal of American Chemical Society 4, 6702-6704, 1970 and in Bulletin de la Société Chimique de France, 945-51, 1964.
Vinyl resins are known to deteriorate under the influence of heat and it is necessary to add a stabilizing agent to these masses of synthetic materials in order to retard thermodegradation and thus delay coloration of the resin.
Vinyl resins are also known to deteriorate under the influence of sunlight and a change in the original coloration may result therefrom.
This problem is particularly important where contain-ers for food and drink are concerned. It is readily appreciated : - 4 -~119176 that food and drink could not be packed in containers of which the coloration may change in course of time.
The problem is the same with regard to floor-coverings, of which the coloration must also remain stable for a long period of time.
That is why stabilizers which are both thermostabil-izers and photostabilizers are specially recommanded for vinyl resins.
Amongst the organic stabilizers there are already known the 1,4-butanediol- bis amino crotona e, the methyl aminocrotonate and the aminocrotonate of alcohols containing from 16 to 18 carbon atoms, compounds which have been described in "Les Matières Plastiques dans l'Industrie Alimentaire" by R. LEFAUX, C.F.E. Paris (1972).
2-Phenyl-indole, however, is especially valuable due both to its good stabilizing power and to its low toxicity.
It is, moreover, widely used in the plastic industry to stabilize vinyl polymers and co-polymers, especially those which are to be used for containing food and drink.
The compounds of the invention have many advantages over the aminocrotonates hereabove mentioned and also over 2-phenyl-indole.
For instance, the compounds of the invention already show a good stabilizing power when they are introduced into a vinyl resin in the proportion of 0.01 to 0.2 part per hundred parts of re~in (p.h.p.r.), whereas the aminocrotonates and 2-phenyl-indole are to be used in a minimum proportion of 0.2 p.h.p~r. Moreover, the compounds of the invention are also superior to 2-phenyl-indole when used in a proportion of 0.2 to 1 p.h.p.r. This means that the stabilizers according to the invention can be used in lower proportions than well-known stabilizers.
_ 5 --~119176 This possibility of using lower proportions of stabilizers is a definite asset as far as plastic containers for food and drink are concerned. Besides the lower cost-price obtained, it should be remembered that when the concentra-tion in stabilizer is lower, the amount of stabilizer extracted by the food or drink will also be lower. -Moreover, the compounds of the invention present a photostabilizing power which is far from negligible resulting in a better resistance of the resin to sunlight, i.e. in a reduced tendency to darken.
In addition, in resins requiring the use of a dye, which is very frequent in bottles for mineral water, the dihydropyridines according to the invention improve the basic colour and markedly increase the stability of the coloration. -~
Finally, the dihydropyridines of the invention possess valuable antioxidant properties, which are superior to those of ;~
the phenols, which are the most widely used antioxidants in vinyl resins (for instance, 2,6-diterbutyl-4-methyl-phenol).
When compared more particularly, to 2-phenyl-indole, the dihydropyridines according to the invention also present ;
the following advantages:
- ~etter transparency of the resin;
- The colorations which are obtained do not affect the trans-parency of the resin and are perfectly stable; '!' ", - Absence of yellowish tinges which are very common in vinyl resins. This enables the use of blue-tinting agents to be avoided;
r - When the dihydropyridines of the invention are used as secondary stabilizers in resins containing as primary stabilizer~ calcium and calcium-zinc salts, they enable, when compared to 2-phenyl-indole used under the same conditions, the content of zinc in the resin to be diminished without .
1~19176 affecting the thermostability of the latter. This is important since it is known that too much zinc causes defects in resin;
- When the compounds of the invention are used as secondary stabiliæers in a plastified resin, i.e. a resin containing as primary stabilizers barium-cadmium and calcium-zinc salts, they enable the content in cadmium, a very expensive and very toxic product, to be diminished while improving appreciably the basic colour and increasing thermostability.
Amongst the compounds of the invention, the stabilizers listed hereunder have been found to be particularly valuable:
2,6-Dimethyl-3,5-dicarboallyloxy-1,4-dihydropyridine (1) 2,6-Dimethyl-3,5-dicarbopropargyloxy-1,4-dihydropyridine (2) 2,6-Dimethyl-3,5-dicarbobenzyloxy-1,4-dihydropyridine (21) 2,4,6-Trimethyl-3,5-dicarbomethoxy-1,4-dihydropyridine (29) The stabilizers of formula I are incorporated in vinyl resins in a proportion of 0.01 to 1 p.h.p.r. and preferably of 0.01 to 0.5 p.h.p.r.
The toxicity of the stabilizers of the invention was studied first and the satisfactory results obtained were such as to justify continuation of the investigation.
A. Acute toxicitv The acute toxicity of the stabilizers of the invention was measured by determining the dose of substance which pro~oked the death of 50% of the treated animals (LD50~.
A LD50 superior to 2g/kg was found in both mice and rats, no toxic symptoms being observed after 15 days.
B. Studv of thermostabilizin~ power The study of the thermostabilizing power of the compounds of the invention was extremely thorough.
Its covers several phases, each of which is important from the point of view of the use of these stabilizers.
1) Statlc thermostability_ - \
11~9~76 This study was carried out according to the GARDNER
method, described in British Patent N 1,489,685.
The reference substance was 2-phenyl-indole, a well-known and widely used stabilizer.
The stabilizer (dihydropyridine or 2-phenyl-indole) and the other usual ingredients were mixed with powdered vinyl resins and the mixtures were calendered at a temperature of 160C to give rigid sheets.
The sheets were then heated in a thermostated oven (185C or 210C) for different intervals of time until incipient carbonization.
The coloration of the samples was compared to a standard scale of coloration, known as the GARD~ER scale and wa~ expressed in terms of the reference figures of the GARDNER
scale.
This study was carried out with the following resin:
Inaredients Parts b~ weiqht Polyvinyl chloride resin 100 Anti-shock resin 9 Epoxide soja bean oil 2 Calcium-12-Hydroxy-stearate 0.2 SL 2016 0.1 Stabilizer 0.3 or 1.55.10 3 mol SL 2016 is a solution of zinc-2-ethyl-hexanoate in a mixture of hydrocarbons boiling between 158C and 184C.
The first trial was carried out at 210C with a resin containing 0.3 part by weight of stabilizer, the sheets being removed from the oven every three minutes over a period of 21 minutes.
The following results were obtained :

1~19176 . . Time in minutes Stablllzer 1 1 1 2 4 7 Burnt 21 1 1 2 35 . 5 14 Burnt .
2-Phenyl-indole 1 2 2 10 11 16 Burnt

4 1 1 2 4 6 11 Burnt 23 1 2 3 13 14 15 Burnt 29 1 2 3 6 7 10 10. 5 Burnt 8 1 2 3 6 8 10. 5 11 Burnt ~-Phenyl-indole 1 2 3 . 510 . 513 14 Burnt 6 1 1 1.5 3 4 7 Burnt 2-Phenyl-indole 1 1 3 8 13 13 Burnt ,....
1 1 2 3 6 10. 5 Burnt 2-Phenyl-indole . 1 3 3 4 11 13 Burnt 22 1 1 2 4 11 Burnt 2-Phenyl-indole 1 2 2 9 11 Burnt :' 3 1 1 3 4 5 10 . 5 Burnt :
2-Phenyl- 1 1 3 510 . 5 11 Burnt , 2 1 1 1 4 8 12 Burnt indole 1 2 3 11 14 18 Burnt 7 1 1 2 410. 5 14 Burnt 2-Phenyl-30indole 1 1 2 6 14 14 Burrt _ 9 _ .: .

~9~76 Time in minutes Stabilizer 17 1 1 2 8 9Burnt 2-Phenyl-indole 1 1 2 11 13 16 Burnt 16 1 l 3 4 ll 18 Burnt 2-Phenyl-indole l l 3 9 14 18 Burnt l 1 1 1 l 4 11 17 2-Phenyl-indole 1 2 2 5 2.5 10 13 16Burnt 26 1 l 1 1 1.5 3 llBurnt :
18 l l 2 2 2 3 6Burnt 1 l 1.5 2 22.5 4 7 2-Phenyl-indole 1 1 1 2 2 4 10.5 11 19 1 1 1 2 3 10 13Burnt 202~PihnednoYle~ 1 1 1 2 4 11 13Burnt 27 1 1 l 2 5 14 Burnt .
2-Phenyl indole 1 1 2 3 5 14 17Burnt 28 l l l 3 4 5 10Burnt 2-Phenyl-indole 1 1 1 3 5 11 13Burnt The above results show a marked superiority of the thermostabilizing power of the dihydropyridines over 2-phenyl-indole, at least up to 12 minutes.
The coloration of the reqins stabilized with compound~
1,2, 4, 5 and 21 is considerably less over the period from 0 to 3 minutes.

-- 10 _ 11~9176 Compounds 1, 2, 3, 16 and 17 have proved to be greatly superior to 2-phenyl-indole.
A second trial was performed at a temperature of 185C, with a resin containing 0.3 part of stabilizer, the samples being removed from the oven every 6 minutes over a period of 24 minutes, then every 3 minutes until the 42nd minute (or until carbonization).
The results given hereunder were obtained:
. -Time in minutes Stabilizer _ _ 21 1 1 2 10 12 12 13 14 Burnt 4 1 1 2 8 13 14 17 18 Burnt 1 1 2 9 11 11 13 14 14 Burnt 6 1 1 2 9 11 11 13 14 Burnt 2-Phenyl-indole 1 1 3 5 13 14 15 15 L6 16 3urnt 1 1 1 3 3 5 7 10 10.5 11 16 Burnt 2 1 1 2 4 6 7 8.5 9 10.5 12 18 2-Phenyl-indole 1 1 4 9 13 14 14 14 14 15 15 _ , i, 3 1 1 5 8 10 10. 13 14 18 18 Burnt 2-Phenyl-indole 1 1 5 10.5 11 13 14 14 15 16 18 _ _ _ The above results taken as a whole show the super-iority of the dihydropyridines over 2-phenyl indole during 30 minutes.
Finally, a trial was performed at 185C, with resins containing equimolecular quantities of the dihydropyridines or of 2-phenyl-indole, namely 1.55 x 10 3 mol. In this case too, the samples were removed from the oven every 6 minutes over a 11~91~i period of 24 minutes and then every 3 minutes until the 42nd minute (or until carbonization).
The following results were obtained :
. . _ . . Time in minutes St abl 11 zer _ _ _ _ _ 21 1 1 1 4 6 8 11 15 Burnt 4 1 1 1 4 6 8 11 15 Burnt 1 1 1 510 11 13 14 15 Burn 6 1 1 2 410 13 s.
2-Phenyl-indole 1 2 3 1013 14 14 14 16 16 Burnt The dihydropyridines, when used in an equimolecular quantity to 2-phenyl-indole, show up to the 30th minute a thermostabilizing power superior to that of this latter compound.
2) Study of tke_duratlon of thermostabilization This study was carried out with vinyl resins which are used for manufacturing bottles for mineral water, namely resin~ containing calcium and zinc stearates, epoxy soja bean oil and either 2-phenyl-indole or a dihydropyridine of the invention (compound 1).
The following trials were carried out with various resins :
1) The resin was mixed in a cylinder-mixer and the following parameters were measured:
- The stability of the colour, i.e. the time which elapsed between the start of the mixing and the first visible change in the colour of the resin.
- The thermostability, i.e. the time which elapsed between the start of the mixing and the carbonization of the resin.
With a resin containing 0.2% of stabilizer, it wa~
found, after mixing at 220C, that the resin containing 2-phenyl-indole as stabilizer changed colour after 3 minutes and 1119~76 carbonized after 13 minutes, whereas the resin containing 2,6-dimethyl-3,5-dicarboallyloxy-1,4-dihydropyridine (Compound 1) changed colour after five minutes and carbonized after 13.3 minutes.
2) Examination of the colour of the resin after each passage (3 in all) in a blower-extruder, the mould being a 250 ml cylindrical flask.
Four resins were tested, the first containing 0.2%
of 2-phenyl-indole and the others 0.2%, 0.03% and 0.015% of compound 1 respectively.
After the first passage the flask presented, with all four resins, a brilliant sky-blue colour.
After the second passa~-e, the flask made with the rèsin containing 2-phenyl-indole presented a blue colour tending towards green, whereas the other three flasks had kept the same colour.
After the third passage, the flask corresponding to the resin stabilized with 2-phenyl-indole had become greenish-blue, whereas the other three flasks still had the same brilliant sky-blue colour.
Two conclusions may be drawn from the above findings:
- In equal concentrations, Compound 1 provides vinyl resin with a stability of colour which is markedly superior to that obtained with 2-phenyl-indole.
- At a concentration of 0.015%, Compound 1 is at all times superior to 2-phenyl-indole used at a concentration of 0.2%.
Trials were also carried out with resins containing a larger quantity of zinc stearate.
The various proportions of calcium and zinc stearate and of the stabilizers are given in the following table:

Additiv ~ Quantity of stabilizer , N of the 1 2 ¦ 3 4 5 6 7 8 9 _ Calcium stearate 0.29 0.29 0.29 0.29 0.29 0.29 0.29 0.29 0.15 Zinc stearate 0.32 0.32 0.32 0.32 0.32 0.32 0.15 _ 0.32 2-Phenyl-indole 0.15 _ _ _ _ _ _ _ _ Compound 1 _ ~ 0 075 0.05 0 03 0 015 ~ 0.15 0.075 The same trials as hereabove were carried out and the following results were obtained (mixing at 220C) Resin in min. 2 to 3 5 4 4 3 3 3 ~1 3 Tbeillmtoyta-in min. 9 8.5 8 8 8 8 15 < 7 5 . _ _ The results show that the improvement of the 8tability of the colour with compound 1 is very good, this latter compound being 10 to 15 times more effective than 2-phenyl-indole.
Compound 1 does not directly influence thermostability but it enables, by preserving the colour, the amount of zinc stearate to be recuded and also thermostability to be increased.
Examination of the colour of the resin after each passage through the blower-extruder, clearly showed the excellent behaviour of the resins containing compound 1.
3) Study of the stabilization of ~lastified vinyl_resins This study was carried out by pressing plates from _ 14 -.

-~9176 sheets which had been mixed for 5 minutes at 180C, the pressing also lasting 5 minutes but at 170C.
The same parameters as hereabo~e were studied, namely the duration of~the stability of the colour, the duration of the thermostability and the colour of the pressed plate.
The following resins were used :

Ingredients Parts by weight r i~ 1 _ ~ - 5 6 ~ _ _ Polyvinyl chloride resin 100 100 100 100 100 100 100 100 100 Wax E 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Dioctylic Phthalate 50 50 50 50 50 50 50 50 50 Barium stearate_ 0.375 i ~ 0.375 0.375 0.375 0.375 0.375 0.375 Cadmium stearate_ _0.375 0.375 0.375 0~15 0.150.050.05 Compound 1 _ ~ _ ~ 0 _ 0.05 _ 0.05 The following results were obtained :

T 'al N of the resin r1 _ 2 3 ¦ 4 5 6 7 6 _ Stability of the colour in min. ~5 <5 25 35 50 30 45 20 35 Thermosta-bility in min. (5 5 25 35 50 30 55 25 50 _ _ _ _ It can be noted that Compound 1 has a favourable influence on the stability of the plate in plastified resin since it :
- incr~aseg the time elapsing between the start of the treatment and the first visible change of colour , 11.~917 Ei - increases the time elapsing between the start of the treatment and the carboniæation of the resin.
Examination of the colour of the various resins shows that the addition of Compound 1 has a favourable influence on the basic colour,lthis latter approaching the colourless.
4) Studv of the stabilization of vinyl co-polymers The study of the static thermostability of vinyl co-polymers was carried out according to the GARDNER method described above and, also in a more detailed manner in British Patent Specification N~ 1,489,685.
a) Rigid co-polymer vinyl chloride-vinyl acetate The following compound was prepared :
Inaredient Parts bv weiqht Co-polymer vinyl chloride-vinyl acetate 80 Vinyl chloride resin 20 Calcium stearate 0.5 Stabilizer 0 or 0.1 The sheets were heated in a ventilated oven at 185~C
and the following results were obtained :

Time in min. _ Stabilize~?~-__ _ _ None 1.5 2 4 11.5 15 17 18 18.5 j28 ~ 1 4 10 12 13 15 17 The above results clearly show the thermostabilizing properties of Compound 28.
b) Flexible co-polymer vinyl chloride-vinyl acetate.
The following compound was prepared :

Inqredient Parts by weiqht Co-polymer vinyl chloride-vinyl acetate 100 Dioctylic phthalate 60 Melamine 2 ; Calcium stearate 2 Stabilizer 0 or 0.2 The resin was allowed to gel for 5 minutes at 120C
and the sheets were heated at 160C in a Metrastat oven.
The following results were obtained :

Time in min.

Stabiliwer ~~~~~
10 None 1 6 13 14 17 2l 1 1 6 12 l5 c) Flexible co-polymer vinyl chloride-vinyl acetate The following compound was prepared :
InqredientParts bv weiaht Co-polymer vinyl chloride-vinyl acetate 100 Dioctylic phthalate 40 Calcium stearate 2 Melamine 2 Stabilizer 0 or 0.3 The sheets were heated in a Metrastat oven at 160C
and the following results were obtained :

j Time in min.

Stabilizer None 1 1 5 8 12 13 13 1 1 1 2 4 5.~ 6 7 .:
. .

! -1~19~7~
d) Rigid co-polymer vinyl chloride-vinylidene chloride The following compound was prepared :
Inaredient Parts bv weiaht Vinyl chloride resin 90 Co-polymer vinyl chloride-vinylidene chloride 10 Anti-shock agent 8 Epoxide soja bean oil 4 Calcium stearate 0.2 Zinc stearate 0.1 hydrogenated rape-seed oil 1.2 Glyceryl trimontanate 0.4 Stabilizer 0 or 0.3 The sheets were heated in a ventilated oven at 210C.
The following results were obtained :
.-Time in min. _ ~ 0 3 6 9 12 15 Stabilizer ~
. ~
None 2 2 5 10.5 14 16 21 1.5 2 2 3 7 15 28 1.' 2 3 3.5 e) Rigid co-polymer vinyl chloride-vinylidene chloride The following compound was prepared :
InaredientParts bv weiaht Vinyl chloride resin 90 Vinyl chloride-vinylidene chloride co-polymer 10 Anti-shock agent 8 Epoxide soja bean oil 4 Calcium stearate 0.2 Zinc stearate 0.1 Hydrogenated rape-seed oil 1.2 Glyceryl trimontanate 0,4 Stabilizer 0 or 0.3 1~19~6 The sheets were heated in a ventilated oven at 210C

and the following results were obtained :
_ . . _ , in min. 0 3 6 9 12 Stabilizer \

~ 1.5 ~ 2.5 ~ 4.5 1 It is clear from tests b, c, d and e that the compounds of the invention have a marked stabilizing action on vinyl chloride co-polymers.

5) Com~arison of the compounds of the invention with ~-amino-crotonate derivatives.
a) Comparison with 1,4-butanediol-~-aminocrotonate and with methyl-~-aminocrotonate.
The following rigid co-polymer was prepared :
Inaredients Parts by weiqht Co-polymer vinyl chloride-vinyl acetate containing 15% of acetate 80 Vinyl chloride polymer 20 Calcium stearate 0,5 Stabilizer 0.5 The sheets were heated in a ventilated oven at 185C and the following results were obtained :
_-me in min.
Stabiliz ~ 0 2 4 6 8 10 12 _ 1,4-butanediol-~-aminocrotonate 1 1 4 10 13 15 18 Methyl-~-amino-crotonate 1 1 5 12 14 17 18 Compound 21 1 1 4 6 11 13 17 Compound 28 1 1 5 7 11 13 17 1J~917~

b) Comparison with stabilizer G 1 Stabilizer G 1 is a mixture of 1,4-butanediol-~-aminocrotonate and of ~-aminocrotonate of alcohols containing from 16 to 18 carbon atoms.
The following rigid co-polymer was prepared :
In~redients Parts bY weiaht Co-polymer vinyl chloride-vinyl acetate 80 containing 15% of acetate Vinyl chloride polymer 20 Calcium stearate 0.5 Stabilizer 0~5 The sheets were heated in a ventilated oven at 185C and the following results were obtained :

. Time in min. _ _ _ i 0 2 4 6 8 10 12 ~ Stabilizer \ .
_ ~ ., G 1 1 1 4 11 12.5 16 18.5 2 1 1 1. 5 10 12 16 G 1 1 1 2. ¦ 11 13.5 17 18.5 . 27 1 1 4 7 12.5 17 _ _ _ _ , Another comparison was made with the following rigid co-polymer :
Inaredients Parts by weiqht Co-polymer vinyl chloride-vinyl acetate containing from 12.5 to 13.5% of acetate 100 Glyceryl tribehenate 0.4 Calcium stearaté 0.5 Stabilizer 0.5 ~,, 30 The sheets were heated in a ventilated oven at 185C
and the following results were obtained :
,j 1119~76 : ~ Time in min. _ _ _ _ _ _ ~ 0 2 ~ 6 8 10 12 14 Stabilizer ~
_ _~ _ G 1 1 2 3 4 9 12 18 _ 2 1 1 2 3 4 8.5 13 17 1 1 1 2 3 7 12 18 _ _ _ A test was also carried out with the following compound :
nqredients Parts by weiqht Co-polymer vinyl chloride-vinyl acetate containing 15% of acetate 80 : Vinyl chloride resin 20 Calcium stearate 0.5 Stabilizer 0-3 The sheets were heated in a ventilated oven at 185~C
and the following results were obtained :
_ e in min 0 2 46 8 10 12 14 Stabiliz ~ _ 20 G 1 1 1 23 6 10 12.516 , ' . _ _ G 1 1 1 26 11.5 16 18Burnt Another test was made with the same resin but containing 0.1 part of stabilizer.
The sheets were also heated in a ventilated oven at 185C and the following results were obtalned :

Time in min. _ _ _ _ _ ~~ O 2 46 8 10 1214 Stabilize~~~--_ _ ~l9~76 G 1 1 1 2 6 13 16 18 _ 21 1 1 2 4 11 13 17 _ c) Comparison with 2-phenyl-indole A comparative test was made using the following compound :
Inqredients Parts by weiqht Co-polymer vinyl chloride-vinyl acetate containing 15% of acetate 80 Vinyl chloride resin 20 Calcium stearate 0.5 Stabilizer 0.3 The sheets were heated in a ventilated oven at 185C
and the following results were obtained :

Time in min. _ Stabilizer ~
_ _ 202-Ph~nyl-indole 1 1 2 3 8 12 14 17 A second test was made using the following compound:
Inqredients Parts by weiaht Vinyl chloride resin 90 Co-polymer vinyl chloride-vinylidene chloride containing 70% of vinylidene chloride 10 Antishock agent 8 Epoxide soja bean oil 4 Calcium stearate 0.2 Zinc stearate 0.1 ~Iydrogenated rape-seed oil 1.2 Glyceryl trimontanate 0.4 1~19176 Stabilizer 0-3 The sheets were heated in a ventilated oven at 210C and the following results were obtained :
_ ..
Time in min.
~~~ O 3 6 9 12 15 Stabilizer ~
_ _ .
2-phenyl-indole 2 2 5 8.5 14 16 21 1.5 2 2 3 7 15 28 1.5 2 3 3.5 7 15 ~ _ , ~
2-phenyl-indole 1 I 4 10 15 _ 1 1 1 2 4 10 _ The above results clearly show that the dihydro-pyridines of the invention have a stabilizing action on vinyl co-polymers which is markedly superior to that of well-known stabilizers, such as the ~-aminocrotonate derivatives and 2-phenyl-indole.
C. Studv of photostabilizinq power -1) Vinyl ~olY_ers_ The photostabilizing power of the compounds of the invention was studied by exposing to the sun polyvinyl chloride plates which had been stabilized either by a dihydropyridine of the invention or by 2-phenyl-indole, or again by 2-(3'-methoxy-4'-hydroxy)-indole. This last product is an extremely valuable thermostabilizer for thermoplastic resins and is described in British Patent N 1,489,685.
The resin given hereunder was prepared by mixing on cylinders at 160C :
InqredientsPart by weiqht Polyvinyl chloride resin 100 ~nti-shock resin 8 l~l9i76 Epoxide soja bean oil 4 Acrylic resin 0-5 ; Trinonylphenyl phosphate 0.3 SL 2016 0.25 Calcium behenate 0.4 Hydrogenated castor oil 0.2 Glyceryl trimontanate 0.4 Stabilizer 0.3 The plates of stabilized polyvinyl chloride, together with a plate which had not been stabilized, were stood in the sun under ~he same conditions and at the ~ame time. Their colour was measured after 6 hours ànd 12 hours of exposure.
The coloration of the plates was determined according to two methods :
- On the plates themselves, following the GARD~ER scale, as - .
in the sect~on on thermostability, , - On a solution of these plates in tetrahydrofuran, the colora-tion of the solution being measured according to the French Pharmacopeia (IXth edition, II-338).

~, 20 The following results were obtained :

Coloration of Coloration of the plate,~ .
the plates FRENCH PHARMACOPEIA
, Stabilizer GARD~ER
Time of exposu-re to sun Time c ,f exposure to sun . . 0 6h 12h 0 6h 12h Standard 3+ 3+ 3+ J5 J5 J5 2-(3'-Methoxy-4'-hydroxy-phe- 1 4 8 B5 B4 ~o JB4 B3 to JB
nyl)-indole _ 2-Phenyl-indole 2 3 5+ J6 J5 J4 Compound 1 1 1 1.5 J6 J5 J5 Compound 2 1 1 1.5 ~V6 JV6 JV5 .
Compound 21 ¦ 1 1 1.5 JV6 ~ JV6 ~ JV5 Compound 29 2 3 5 JV6 JV5 JV4 The sign + means that the coloration lies between the lower unit and the upper half-unit. Thus, 3~ means that the coloration lies between 3 and 3.5. Similarly, the sign - means that the coloration lies between the lower half-unit and the upper unit.

It may be concluded from the above results that the -dihvdropyridines according to the invention have a photo-stabilizing power on vinylpolymers which is markedly superior to that of 2-phenyl-indole and of its derivative.
2) Vinyl _o-polYmers The sheets were exposed to the sun and their colora-tions were also determined according to the GARDNER method.
a) Co-polymer vinyl chloride-vinyl acetate The following compound was prepared :
InaredientsParts ~y we'iqht Co-polymer vinyl chloride-vinyl acetate conbaining 15% of acetate 100 Dioctylic phthalate 40 Calcium ~tearate 2 Melamine 2 Wax Stabilizer 0.3 The following results were obtained :

Time in hours Colour in GARDNER degrees Stabilizer Time 0 After 24 hours 2-Phenyl-indole 1 6 ~, . `' :

~` 1119~76 .
-~b~ L-indole ~ 1 ~

b) Rigid co-polymer vinyl chloride-vinylidene chloride The following compound was prepared :
Inqredients Parts by weiqht 10Vinyl chloride resin 90 Co-polymer vinyl chloride-vinylidene chloride containing 70% of vinylidene chloride lO
Anti-shock agent 8 Epoxide soja bean oil 4 Calcium stearate 0.2 Zinc stearate 0.1 Hydrogenated rape-seed oil 1.2 Glyceryl trimontanate 0.4 Stabilizer 0.3 The following results were obtained:

Time in hours Colour in GARD~ER degree Stabilizer Time 0 After 24 hours 21 1 5 3.5 :;~
28 1.5 3.5 . _ 2-Phenyl-indole 1 13 The above results clearly show the marked superiority of the compounds of the invention over 2-phenyl-indole with regard to the stabilization of vinyl co-polymers.
D. Study of antioxidant Power ... ~ :

~9176 The value of the antioxidant power of the 1,4-dihydropyridines o~ the invention was demonstrated by two differents methods :
- ~y a polarographic study of their oxidation potential, compared to well-known antioxidants such as 2,6-diterbutyl-4-methyl-phenol, 2-terbutyl-4-methoxy-phenol, 4-methoxy-phenol and hydroquinone.
- By direct comparison, on a polyvinyl chloride resin, of the anti oxidant power of Compound 1 of the invention and that of an anti-oxidant known as one of the most valuable and frequently used in polyvinyl chloride, namely 2,6-diterbutyl-4-methyl-phenol.
1) Polaroqraphic studY of the oxidation potential a) O~eratina conditions E_e=c=t==des Reference electrode : Calomel electrode containing a saturated solution of anhydrous lithium perchlorate as bridge liquid.
Workina electrode : Vitrous carbon rotating electrode (2,500 r. p.m.) Counter-electrode : Platinum electrode =he-m=Lca=l=~2=od=u=c=t==
- Acetonitrile containing less than 0.1% of water and presenting no polarographic waves between -2.5 V and +2.5 V.
- Anhydrous lithium perchlorate containing less than 1%
of water.
Re=a=ct=nt A 0.1 M solution of lithium perchlorate in acetonitrile, treated and preserved on a 4 A molecular sieve.
Pg_==~o~==pb-= ==o=n=d=t===o=n_ Tension : 10 m V

Initial potential : 0 V
Amplitude of exploration : 0 to 2 V
Speed of exploration : 10 a V/second Sensitivity : 1.25 ,u A to 50 ,u A
Mean concentration : about 0.3 x 10 3 mol/litre Precautions Between each measurement, the vitrous carbon electrode and the platinum electrode were carefully cleaned with Joseph paper.
Results Substance Oxidation potential in volts 1 0.75 + 0.01 21 0.73 + 0.01 2 0.80 + 0.01 2,6-Diterbutyl-4-methyl-phenol 1.11 + 0.02 2-Terbutyl-4-methoxy-phenol 0.81 + 0.01 Hydroquinone 0.83 + 0.01 ' 4-Methoxy-phenol 0.89 + 0.01 The above results show that the dihydropyridines are more reducing than the substance usually used as anti-oxidants.
2) Trial on a polyvinyl chloride resin The antioxidant power of Compound 1 was compared to that of 2,6-diterbutyl-4-methyl-phenol (Compound 2) by using the following resin :
, Inaredients Parts by weiqht Polyvinyl chloride resin 100 Anti-shock agent 10 Acrylic resin 0,5 Epoxide soja bean oil 3 SL 2016 0.1 Zinc and calcium stearates 0.2 "` lli~1~6 Hydrogenated castor oil 1.5 Polyethylene wax 0.3 Antioxidant from 0.05 to 1 The study was carried out according to the GARD~ER
method, the samples being removed from an oven at 185C every 10 minutes over a period of 80 minutes.
The following results were obtained :

!Concentra- Anti- Tim~ ! ln minutes _ prartls oxidant 0 0 20 30 40 50 60 7080 0.05 Compound Z ~1 1+ 2 5+ >6 7 7 83urnt Compound 1 1 1 1+ 3+ 4 6+ 7 8 B
. _ _ 0.1 Compound Z >11+ 2 ~5 ~6 7 7 8 B
Compound 1 1 1 1+ <2 3+ 5+ 7 7 B

0.2 Compound Z ~ 11+ 2+ 5 ~6 7 7 8 B
, Compound 1 1 1 1 1+ 2 3 5 ~6 B
,; _ _ _ 0.3 Compound Z ~1+ 1+ ~2 6 78 8 8+ B
Compound 1 1 1 1 1 2~2 8 8 B

0.4 Compound Z ~1+ >1+ 3 ~6 89 9 9 B

Compound 1 1 1 1 1 1+ ~2+ 4 8 B
_ _ _ _ _ 0.5 Compound Z <1+ ~1+ 3~6 >7 8 8 8 B

Compound 1 1 1 1>1 1+ ~2+ 4 8 B
_ 1.0 Compound Z 1+ 2 ~38 8 8 9 9 B

Compound 1 1 1 11 1+ 4 8 B -The signs + and - have the same meanings as in the previous table and > 2 means that the coloration lies between 2 and 2+.
The following conclusions can be drawn from the above table : up to 50 minutes in the oven at 185C , Compound 1 1~1917~;

proved to be superior to Compound Z at every concen~ration.
Moreover, no concentration in Compound Z gives the best results obtained with Compound 1.
The same trials were carried out in a Metrastat oven at 210C.
The following results were obtained :
Concentra . Time in minutes tion part Antl-per 100 oxidant0 to 10 to 20 to 25 tc 30 to 35 to parts 10 20 25 30 35 40 Compound Z 2 2 3 4+ 10 ~ 10 0.05 B/38' Compound 1 1 1 1 1 5 B~/4~' 0.1 Compound Z 2 2 3 4+ 11 B~/39' Compound 1 1 1 1 1 5 BC/3l8, Compound Z 2 2 3 4+ 11 ~ 11 o.2 B/39' Compound 1 1 1 1 1 5 ~ 9 B/39' 0.3 Compound Z ~ 1 2 3 3 9 B~/309' . Compound 1 1 1 1 1 3+ ~ 9 ~/19' Compound Z 2 2 3 3+ 10 > 10 0.4 B/38' Compound 1 1 1 1 1 4 6-B/38' _ 0.5 Compound Z 2 2 3 3+10+ B/38' Compound 1 1 1 1 1 3+ B/38 Compound Z 2 2 3 5 11 > 11 1.0 B/38' Compound 1 1 1 1 1 2+ > 2+
_ B/37' As in the previous trial, Compound 1 was found to be markedly superior to Compound Z. Moreover, all the samples containing Compound Z gave, from the time zero, pink-coloured sheets, which is another disadvantage as compared to the dihydropyridine of the invention.
The following Examples provide a non-limitative illustration of the process of preparation of the compounds of the invention.

PreDaration of 2 6-dimethyl-3,5-dicarboallyloxy-1,4-dihydro-Pvridine Into a reactor cooled in an ice-bath were introduced 28.4 g (0.2 mol) of allyl acetoacetate and 0.2 g of diethyl-amine.
The solution was cooled to 0C and 7.5 g (0.1 mol) of a 40% aqueous solution of formic aldehyde was introduced drop-by-drop in 90 minutes, care being taken to maintain the temperature below or equal to 10C.
The reaction medium was maintained at 0C for 6 hours, and then at room-temperature for 40 hours.
The solution was decanted and the aqueous phase was extracted with ether. The ethereal phase was added to the oily phase and was dried over anhydrous sodium sulphate.
The solution was filtered, the ether eliminated and the oily residue diluted with one part of methanol.
Ammonia was bubbled through the solution obtained, the temperature heing maintained at 0C, and the solution saturated with ammonia was allowed to stand at room-temperature for 12 hours.
The solution was filtered on a Buchner funnel and the solid residue so obtained was recrystallized from acetone to give 2,6-dimethyl-3,5-dicarboallyloxy-1,4-dihydropyridine.

~19~76 Yield : 75%.
Melting point : 167C.
By the method described above but using the appropriate starting-products, the following compounds were prepared :
Compound Yield % Meltina point C
2,6-Dimethyl-3,5-dicarbopropar-gyloxy-1,4-dihydropyridine 60202-203 (acetone) 2,6-Dimethyl-3,5-di-(2'-carbochloro-ethoxy)-1,4-dihydropyridine 31 153 (acetone) 2,6-Dimethyl-3,5-di(4'-carbomethoxy-benzyloxy)-1,4-dihydropyridine 18 143 (benzene 55~C) 2,6-Dimethyl-3,5-dicarbobenzyloxy-1,4-dihydropyridine 57 122 (benzene) 2,6-Dimethyl-3,5-dicarbocinnamyloxy-1,4-dihydropyridine 37 156 (benzene then acetone) 2,6-Dimethyl-3,5-di-(2'-carbochloro-benzyloxy)-1,4-dihydropyridine 44 190 (acetone) 2,6-Dimethyl-3,5-di-(2'-carbomethyl-benzyloxy)-1,4-dihydropyridine 47 149 (acetone) Preparation of 2,6-dimethyl-3 5-dicarboproparqyloxy-1l4-dihydropYridine Into a reactor were introduced 14g (0.1 mol) of propargyl acetoacetate, 10.5g (0.075 mol) of tetramine hexa-methylene, 2.9g (0.033 mol) of ammonium acetate, 36g of methanol and 5g of water.
Under a light flow of nitrogen, the reaction medium was refluxed for one hour and was allowed to cool to room-temperature and was then poured into a mixture of water and ice.

1~19176 The precipitate which formed was filtered off and was washed with hot acetone in order to eliminate the excess of tetramine hexamethylene.
The precipitate was dried to constant weight and washed with hot acetone to give 2,6-dimethyl-3,5-dicarbopropar-gyloxy-1,4-dihydropyridine.
Yield : 75%
Melting Point : 206C.
By the above procedure but using the appropriate starting-product, the following compounds were prepared :

Compound Yield % Meltinq Point C
2,6-Dimethyl-3,5-dicarboallyloxy-1,4-dihydropyridine 86 168 (washing with hot acetone) 2,6-Dimethyl-3,5-di-(4'-carbochloro-benzyloxy~l,4-dihydropyridine 82 171 (acetone) ~`
2,6-Dimethyl-3,5-dicarbobenzyloxy-1,4-dihydropyridine 75 120 (methanol) 2,6-Dimethyl-3,5-di-(2'-carbobutoxy-ethoxy)-1,4-dihydropyridine 40 95 (methanol) 2,6-Dimethyl-3,5-di-(2'-carbometho~y-ethoxy)-1,4-dihydropyridine 70 107 (methanol) 2,6-Dimethyl-3,5-di-(2'-carboethoxy-ethoxy)-1,4-dihydropyridine 65 109 (methanol then acetone) 2,6-Dimethyl-3,5-di-(2'-carbophenoxy-ethoxy)-1,4-dihydropyridine 55 132 (acetone) 2,6-Diphenyl-3,5-dicarbethoxy-1,4-dihydropyridine 10 142 (ethylic ether) 2,6-Dibenzyl-3,5-dicarbethoxy-1,4-dihydropyridine 51 118 ` (methanol) 2,6-Di-3-butenyl-3,5-dicarbomethoxy-1,4-dihydropyridine 46 79 (benzene/pentane 10/30) 2,6-Dimethyl-3,5-di-(2"-carbochloro-2'-ethoxy-ethoxy)-1,4-dihydropyridine 79 110 (methanol) 2,6-Diphenylethyl-3,5-dicarbomethoxy-1,4-dihydropyridine 59 156 (methanol-acetone 80/20) 2,6-Ditridecyl-3,5-dicarbomethoxy-1,4-dihydropyridine 73 61 (acetone) Preparation of 2,4 6-trimethyl-3,5-dicarbomethoxv-1,4-dihydropvridine Into a reactor were introduced 232g (2 mols) of methyl acetoacetate and 160g of methanol. While stirring, 44g (1 mol) of freshly distilled acetaldehyde were added drop-by-drop, at 15C.

Temperature and stirring were maintained for 15 minutes under a light flow of nitrogen and 90g of a 20%
ammonium hydroxide solution (l.lS mol) were added at room-temperature. The temperature rose to 45C and the mixture wa~ heated to the methanol reflux temperature for one hour.
After cooling, the reaction medium was poured into a water-ice mixture and the precipitate which formed was filtered off on a Buchner funnel.
The precipitate wa~ thoroughly washed with water and wa9 recrystallized from methanol to give 2,4,6-trimethyl-3,5-dicarbomethoxy-1,4-dihydropyridine.

Yield : 69 %
Melting point : 155-156C.
By the above procedure but using the appropriate starting-product, the following compounds were prepared :
Compound ~ield % Meltinq Point C
2,6-Dimethyl-3,5-dicarbomethoxy-4-hexyl-1,4-dihydropyridine 70 100 (cyclohexane) 2,6-Dimethyl-3,5-dicarbomethoxy-4-propyl-1,4-dihydropyridine 57 140 (acetone) ; - 2,4,6-Trimethyl-3,5-di-(4'-carbo-methylphenoxy)-1,4-dihydropyridine 20 192 (acetone) 2,4,6-Trimethyl-3,5-dicarbobenzy-loxy-1,4-dihydropyridine 61 123 (methanol) 2,4,6-Trimethyl-3,5-dicarboallyloxy- ;
1,4-dihydropyridine 55 100 (benzene-hexane) 2,4,6-Trimethyl-3,5-di-(4'-carbochloro-benzyloxy)-1,4-dihydropyridine28 161 (methanol) 2,4,6-Trimethyl-3,5-dicarbophenoxy-1,4-dihydropyridine 5 186 (methanol) 2,6-Dimethyl-3,5-dicarbomethoxy-4-phenyl-1,4-dihydropyridine 55 194 (acetone) Pre~aration of 2,6-dimethvl-3.5-dicarbomethoxY-4-(1-propenYl)-1, 4-dihvdropYridine Into a reactor were introduced at room-temperature 23.2g (0.2 mol) of methyl acetoacetate and 7g (0 r 1 mol) of crotonaldehyde. While stirring, a solution of 40g (0.42 mol) of ammonium carbonate in 400g of water was added and stirring - .

~19~76 was maintained ~or 20 hours at room-temperature.
The gummy precipitate was coled by ether and the ethereal solution was washed with water, dried over sodium sulphate and the ether was evaporated off.
The crude product was recrystallized from a mixture of ethanol and hexane to give 2,6-di~ethyl-3,5-dicarbomethoxy-4-(1-propenyl)-1,4-dihydropyridine.
Yield : 60%
Melting point : 176C.

'

Claims (18)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. 3,5-Dicarboxy dihydropyridine derivatives having the general formula:
I

wherein the radicals R1 are the same and each represents a branched- or straight-chain alkyl group containing from 1 to 16 carbon atoms, a butenyl radical, a phenyl radical or a benzyl radical, the radicals R2 are the same and each represents a branched- or straight-chain alkyl group con-taining from 1 to 4 carbon atoms, an allyl radical, a propargyl radical, an .omega.-halogenoethyl group, a benzyl radical, a benzyl radical substituted on the ortho- or para-position by a halogen atom or a methyl radical, a cinnamoyl radical, a phenyl radical, a phenyl radical substituted on the para-position by a halogen atom or a methyl radical, a radical of the formula R4-O-(CH2)n-, wherein n is 1, 2, 3 or 4 and R4 represents a branched- or straight-chain alkyl group con-taining from 1 to 4 carbon atoms, unsubstituted or substituted on the .omega.-position by a halogen atom, or R4 represents a phenyl group, and R3 represents a hydrogen atom or a branched-or straight-chain alkyl group containing from 1 to 7 carbon atoms, with the proviso that when R1 represents a methyl radical and R3 a hydrogen atom, R2 does not represent an alkyl or a phenyl radical, and excluding:
2,6-dimethyl-3,5-dicarbobenzyloxy-1,4-dihydropyridine, 2,6-dimethyl-3,5-dicarbocinnamyloxy-1,4-dihydropyridine, 2,6-dimethyl-3,5-dicarbomethoxy-4-phenyl-1,4-dihydropyridine, 2,6-diphenyl-3,5-dicarboethoxy-1,4-dihydropyridine, 2,6-dimethyl-3,5-di-(2'-carbomethoxy-ethoxy)-l,4-dihydropyri-dine, 2,6-dimethyl-3,5-di-(2'-carboethoxy-ethoxy)-1,4-dihydropyri-dine, 2,6-dimethyl-3,5-di-(2'-carbobutoxy-ethoxy)-,4-dihydropyridine, 2,6-dimethyl-3,5-di-(2'-carbophenoxy-ethoxy)-1,4-dihydropy-ridine, and 2,4,6-trimethyl-3,5-dicarbomethoxy-1,4-dihydropyridine.

2. Derivatives according to claim 1, wherein R3 represents a hygrogen atom.

3. 2,6-Dimethyl-3,5-dicarboallyloxy-1,4-dihydro-pyridine.

4. 2,6-Dimethyl-3,5-dicarbopropargyloxy-1,4-dihydropyridine.

5. Process for the preparation of the compounds of general formula I:
(I) wherein the radicals R1 are the same and each represents a branched- or straight-chain alkyl group containing from 1 to 16 carbon atoms, a butenyl radical, a phenyl radical or a benzyl radical, the radicals R2 are the same and each represents a branched- or straight-chain alkyl group con-tainlng 1 to 4 carbon atoms, an allyl radical, a propargyl radical, an .omega.-halogenoethyl group, a benzyl radical, a benzyl radical substituted on the ortho- or para-position by a halogen atom or a methyl radical, a cinnamoyl radical, a phenyl radical, a phenyl radical substituted on the para-position by a halogen atom or a methyl radical, a radical of the formula R4-O(CH2)n-, wherein n is 1, 2, 3 or 4 and R4 represents a branched- or straight-chain alkyl group con-taining from 1 to 4 carbon atoms, unsubstituted or substituted on the .omega.-position by a halogen atom, or R4 represents a phenyl group, and R3 represents a hydrogen atom or a branched-or straight-chain alkyl group containing from 1 to 7 carbon atoms, with proviso that when R1 represents a methyl radical and R3 a hydrogen atom, R2 does not represent an alkyl or a phenyl radical, wherein a compound of the general formula:
(II) wherein R1 and R2 have the same meanings as in claim 1, is reacted with an aldehyde of the general formula:
(III) Wherein R3 has the same meanings as in claim 1, and with ammonia.

6. Process according to claim 5, wherein use is made of an aldehyde of the formula (III) in which R3 represents a hydrogen atom.

7. Process for stabilizing a vinyl resin, wherein a compound of general formula I:
(I) wherein the radicals R1 are the same and each represents a branched- or straight-chain alkyl group containing from 1 to 16 carbon atoms, a butenyl radical, a phenyl radical or a benzyl radical, the radicals R2 are the same and each represents a branched- or straight-chain alkyl group con-taining from 1 to 4 carbon atoms, an allyl radical, a propargyl radical, an .omega.-halogenoethyl group, a benzyl radical, a benzyl radical substituted on the ortho- or para-position by a halogen atom or a methyl radical, a cinnamoyl radical, a phenyl radical substituted on the para-position by a halogen atom or a methyl radical, a radical of the formula R4-O-(CH2)n-, wherein n is 1, 2, 3 or 4 and R4 represents a branched- or straight- chain-alkyl group containing from 1 to 4 carbon atoms, unsubstituted or substituted on the .omega.-position by a halogen atom, or R4 represents a phenyl group, and R2 represents a hydrogen atom or a branched- or straight-chain alkyl group containing from 1 to 7 carbon atoms, with proviso that when R1 represents a methyl radical and R3 a hydrogen atom, R2 does not represent an alkyl or a phenyl radical, is introduced into a polymer or copolymer of vinyl chloride.

8. Process according to claim 7, wherein, in the compound of the formula I, R3 represents a hydrogen atom.

9. Process according to claims 7 or 8, wherein the compound of the general formula I is introduced into the vinyl resin in a proportion of 0.01 to 1 part by 100 parts of resin by weight.

10. Process according to claims 7 or 8, wherein the compound of the general formula I is introduced into the vinyl resin in a proportion of 0.01 to 0.5 part by l00 parts of resin by weight.

11. Process according to claims 7 or 8, wherein the vinyl resin is polyvinyl chloride.

12. Process according to claims 7 or 8, wherein the vinyl resin is a vinyl chloride-vinyl acetate copolymer.

13. Process according to claims 7 or 8, wherein the vinyl resin is a vinyl chloride-vinylidene chloride copolymer.

14. Polymers and copolymers of vinyl chloride containing as stabilizer at least one compound of general formula I:
wherein the radicals R1 are the same and each represents a branched- or straight-chain alkyl group containing from 1 to 16 carbon atoms, a butenyl radical, a phenyl radical or a benzyl radical, the radicals R2 are the same and each repre-sents a branched- or straight-chain alkyl group containing from 1 to 4 carbon atoms, an allyl radical, a propargyl radical, an .omega.-halogenoethyl group, a benzyl radical, a benzyl radical substituted on the ortho- or para-position by a halogen atom or a methyl radical, a cinnamoyl radical, a phenyl radical, a phenyl radical substituted on the para-position by a halogen atom or a methyl radical, a radical of the formula R4-O(CH2)n-, wherein n is 1, 2, 3 or 4 and R4 represents a branched- or straight-chain alkyl group containing from 1 to 4 carbon atoms, unsubstituted or substituted on the .omega.-position by a halogen atom, or R4 represents a phenyl group, and R3 represents a hydrogen atom or a branched- or straight-chain alkyl group containing from 1 to 7 carbon atoms with proviso that when R1 represents a methyl radical and R3 a hydrogen atom, R2 does not represent an alkyl or a phenyl radical.

15. Polymers and copolymers of vinyl chloride according to claim 14, wherein said at least one compound of general formula I as defined in claim 14, is contained in a proportion of 0.01 to 1 part by 100 parts of resin by weight.

16. Polymers and copolymers of vinyl chloride according to claim 14, wherein said at least one compound of general formula I as defined in claim 14, is contained in a proportion of 0.01 to 0.5 part by 100 parts of resin by weight.

17. Polymers and copolymers of vinyl chloride according to claim 14, further containing calcium and calcium-zinc salts, as primary stabilizers, said at least one compound of general formula I acting as secondary stabilizer.

18. Plastified polymers and copolymers of vinyl chloride aceording to claim 14, further containing barium-cadmium and calcium-zinc salts as primary stabilizers, said at least one compound of general formula I acting as secondary stabilizers.