JPH09327625A - Lactide catalyst and preparation of lactide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To depolymelyze a lactic acid olygomer at a low temperature to prepare a lactide having a high optical purity at a high chemical yield by using a tin compound having a specified formula as a catalyst. SOLUTION: This lactide catalyst for a lactic acid olygomer is represented by formulas, I, II, III, IV. In the formulas I, II, III, R1 and R2 each represent a saturated alkyl group, X1 -X3 and Y each represent a halogen atom, hydroxy group, a acyloxy group derived from a fatty acid having a carbon number up to 18 and an alkyl sulfonyloxy group or alylsulfonyloxy group. In the formula III, R31, R32, R33, R34 each represent a saturated alkyl group or aryl group. In the formula IV, R41, R42, R43, R44 each represent a saturated alkyl group and Z1, Z2 each represent a halogen atom, hydroxy group, an acyloxy group derived from a fatty acid having a carbon number of up to 18, a tiocyanate group, an alkylsulfonyloxy group, or arylsulfonyloxy group. At least one of catalysts represented by these formulas I-IV is used.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a catalyst for producing lactide, which is a cyclic dimer of lactic acid, by depolymerizing a lactic acid oligomer, and a method for producing lactide using the catalyst. The present invention relates to a lactide-forming catalyst and a method for producing lactide, which are capable of producing lactide with optical purity in a high chemical yield.

[0002]

2. Description of the Related Art Polylactic acid is a biodegradable plastic with a low environmental load, as compared with conventional plastics such as polyethylene, polypropylene, polystyrene, polyethylene terephthalate, and vinyl chloride, in the medical field, agricultural field, food packaging field, and other fields. It is expected to be used in a wide range of industries. That is, polylactic acid is hydrolyzed and decomposed by microorganisms in the natural environment, and finally decomposes into carbon dioxide and water. Also, even if incinerated,
Because the calories burned are small, it does not damage the incinerator,
No harmful gas is generated. Therefore, the problem of waste disposal in conventional plastics is greatly reduced. Also,
Since polylactic acid uses lactic acid obtained from renewable plant resources as a raw material, it is more promising in terms of resources in the future than conventional plastics are synthesized from petroleum raw materials. Further, polylactic acid is characterized by having higher transparency than other biodegradable polymers.

Polylactic acid can be obtained by directly dehydrating and condensing a lactic acid monomer, or by once synthesizing a cyclic lactide which is a dimer of lactic acid and then subjecting the lactide to ring-opening polymerization.

Conventionally, as a method for producing lactide, lactic acid is heated under reduced pressure to give a lactic acid oligomer (or polylactic acid), and then the lactic acid oligomer (or polylactic acid) is heated under reduced pressure in the presence of a catalyst to depolymerize. There is a way to convert to lactide by doing. Many studies are known for this lactide production method. For example, German Patent 1083
No. 275 describes a method for producing lactide from polylactic acid in the presence of a metal of Group VI, Group V or Group VIII of the Periodic Table or a salt thereof, such as lead oxide, tin oxide or antimony oxide. . German Patent No. 3708915 describes a process catalyzed by tin and its compounds. Further, in German Patent No. 1234703, a titanium tetraalkoxide catalyst is disclosed in German Patent No. 250.
No. 413 uses zinc oxide catalysts respectively. However, in any of the above-mentioned methods, since the catalyst is a heterogeneous catalyst, the chemical yield of lactide is not satisfactory.

On the other hand, since the organic tin compound is soluble in the ester, it is effective as a homogeneous catalyst for high viscosity polyester compounds. For example, JP-A-63-101378
The publication describes a method for producing lactide from lactic acid or polylactic acid in the presence of an organotin compound derived from metallic tin (II), tin (II) halide and carboxylic acid. However, the chemical yield is not so high due to the deactivation of the catalyst, which is thought to be due to the oxidation of divalent tin (65-70).
%).

Furthermore, as an organotin catalyst for the depolymerization reaction, trifluoromethanesulfonic acid (JP-A-5-105) is used.
745), dibutyltin (JP-A-6-31175).
Japanese Patent No. 4) is used. However, although the trifluoromethanesulfonic acid catalyst gives a high chemical yield, it requires a high reaction temperature of, for example, 170 to 250 ° C., so that there is a problem that the optical purity of the obtained lactide is low. That is, it is considered that the high temperature at the time of depolymerization of the lactic acid oligomer causes inversion of the asymmetric carbon atom to lower the optical purity of lactide. Also, the chemical yield of dibutyltin catalyst is not so high (reaction temperature 16
There is a problem of about 50-60% at 0 ° C).

[0007]

Therefore, an object of the present invention is to produce a lactide-forming catalyst and lactide which can produce lactide having a high optical purity in a high chemical yield by depolymerizing a lactic acid oligomer at a relatively low temperature. To provide a method.

[0008]

As a result of extensive studies, the present invention found that a tin compound having a specific structure is excellent as a depolymerization catalyst for a lactic acid oligomer, and completed the present invention.

That is, the lactide-forming catalyst for lactic acid oligomer of the present invention comprises the following general formula (I), general formula (II) and general formula (I
II) or general formula (IV).

[0010]

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In the general formula (I), R ₁ represents a saturated alkyl group, and X ₁ , X ₂ and X ₃ may be the same or different and may be a halogen atom, a hydroxy group or a fatty acid having up to 18 carbon atoms. Represents a derivatized acyloxy group, alkylsulfonyloxy group or arylsulfonyloxy group.

[0012]

[Chemical 6]

In the general formula (II), R ₂ represents a saturated alkyl group, Y represents a halogen atom, a hydroxy group, an acyloxy group derived from a fatty acid having up to 18 carbon atoms, an alkylsulfonyloxy group or an arylsulfonyloxy group. Represents

[0014]

[Chemical 7]

In the general formula (III), R ₃₁ , R ₃₂ ,
R ₃₃ and R ₃₄ may be the same or different and each represents a saturated alkyl group or an aryl group.

[0016]

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In the general formula (IV), R ₄₁ , R ₄₂ and R
₄₃ and R ₄₄ may be the same or different and represent a saturated alkyl group, and Z ₁ and Z ₂ may be the same or different and are derived from a halogen atom, a hydroxy group or a fatty acid having up to 18 carbon atoms. Represents an acyloxy group, a thiocyanate (—NCS) group, an alkylsulfonyloxy group or an arylsulfonyloxy group. In addition, the general formula (I
The compound of V) may form a dimer.

Further, the method for producing lactide of the present invention is a method for producing lactide by heating a lactic acid oligomer under reduced pressure in the presence of a catalyst to depolymerize it, and using the above-mentioned general formula (I) or general formula (I) as a catalyst. II), general formula (III) or general formula (IV)
It is characterized in that at least one of the lactide-forming catalysts represented by

The present invention will be described in detail below. First, the general formula (I) will be described in detail.

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In the general formula (I), R ₁ represents a linear or branched saturated alkyl group. The carbon number of the alkyl group is not particularly limited, but it is usually 1 to 17 carbon atoms. Those having such a carbon number have good solubility at the time of depolymerization of the lactic acid oligomer, and a uniform reaction can be efficiently performed. The preferred carbon number is 4 to 17, and the more preferred carbon number is 4 to 8. Specific examples of the saturated alkyl group for R ₁ include, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, pentyl, hexyl, heptyl, octyl, 2-ethylhexyl, nonyl and decyl groups. Etc. Of these,
Preferred are n-butyl, isobutyl, sec-butyl, pentyl, hexyl, heptyl, octyl and 2-ethylhexyl groups.

X ₁ , X ₂ and X ₃ may be the same or different and each is a halogen atom, a hydroxy group, an acyloxy group (—OCOR) derived from a fatty acid (RCOOH) having up to 18 carbon atoms, or an alkylsulfonyloxy group. Represents a group or an arylsulfonyloxy group.

Examples of the halogen atom include a fluorine atom, a chlorine atom and a bromine atom. Among these halogen atoms, a chlorine atom is preferable from the viewpoint of easy production and handling of the compound.

Acyloxy groups having up to 18 carbon atoms (--OC
Examples of OR) include acetic acid (R: CH ₃ ), propionic acid (R: C ₂ H ₅ ), butyric acid (R: C ₃ H ₇ ), valeric acid (R: C ₄ H ₉ ), caproic acid (R) R: C ₅ H ₁₁ ), enanthate (R: C ₆ H ₁₃ ), caprylic acid (R: C
₇ H ₁₅ ), pelargonic acid (R: C ₈ H ₁₇ ), capric acid (R: C ₉ H ₁₉ ), undecyl acid (R: C ₁₀ H ₂₁ ), lauric acid (R: C ₁₁ H ₂₃ ), tridecyl Acid (R: C ₁₂ H
₂₅ ), myristic acid (R: C ₁₃ H ₂₇ ), pentadecyl acid (R: C ₁₄ H ₂₉ ), palmitic acid (R: C ₁₅ H ₃₁ ), heptadecyl acid (R: C ₁₆ H ₃₃ ), stearic acid ( R: C
₁₇ H ₃₅ ) and the like.
Of these, an acetoxy group is preferable from the viewpoint of ease of production, but it has a drawback that it is easily hydrolyzed, and therefore a capryloxy group, a lauryloxy group, and a stearyloxy group are preferable from the viewpoint of solubility of the catalyst. Is also preferable.

The alkylsulfonyloxy group generally includes an alkylsulfonyloxy group having 1 to 4 carbon atoms, and specific examples thereof include a methanesulfonyloxy group, an ethanesulfonyloxy group and an n-propanesulfonyloxy group. . Of these, a methanesulfonyloxy group is preferred from the viewpoints of ease of production and availability.

Examples of the arylsulfonyloxy group include a substituted or unsubstituted phenylsulfonyloxy group, and specific examples include a phenylsulfonyloxy group, a toluenesulfonyloxy group, a naphthylsulfonyloxy group and the like. Of these, a toluenesulfonyloxy group is preferable from the viewpoint of ease of production and availability.

Preferred compounds of general formula (I) are R
₁ : a compound of n-butyl group, X ₁ , X ₂ , X ₃ : Cl;
R ₁ : n-butyl group, X ₁ : Cl, X ₂ , X ₃ : a compound of a hydroxy group; R ₁ : n-butyl group, X ₁ , X ₂ : C
l, _{X 3:} a compound of a hydroxy group; _{R 1:} n-butyl _{group, X 1, X 2, X} 3: a compound of -OCOC ₇ H _15;
R _1: n-octyl _{group, X 1, X 2, X} 3: -OCOC 9
Examples thereof include H ₁₉ compounds.

Next, the general formula (II) will be described in detail.

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In the general formula (II), R ₂ represents a linear or branched saturated alkyl group, and R ₂ in the general formula (I) is
Synonymous with ₁ . Y represents a halogen atom, a hydroxy group, an acyloxy group (—OCOR) derived from a fatty acid (RCOOH) having up to 18 carbon atoms, an alkylsulfonyloxy group or an arylsulfonyloxy group, and X in the general formula (I) It is synonymous with _{1 to} X ₃ .

Preferred compounds of the general formula (II) are R
₂ : a compound of n-butyl group, Y: hydroxy group; R ₂ :
n- butyl group, Y: a compound of acetoxy group; _{R 2:} n- butyl group, Y: compounds of -OCOC ₇ H ₁₅ and the like.

Next, the general formula (III) will be described in detail.

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In the general formula (III), R ₃₁ , R ₃₂ ,
R ₃₃ and R ₃₄ may be the same or different and each represents a linear or branched saturated alkyl group or an aryl group.

The saturated alkyl group has the above-mentioned general formula.
The same as the saturated alkyl group represented by R ₁ in (I) can be mentioned.

Examples of the aryl group include a substituted or unsubstituted phenyl group, and specific examples include a phenyl group, o-, and m.
-, P-toluyl group, o-, m-, p-methoxyphenyl group and the like. Of these, from the viewpoint of manufacturing ease,
A phenyl group is preferred.

Preferred compounds of general formula (III) are:
R ₃₁ , R ₃₂ , R ₃₃ , R ₃₄ : phenyl group compound; R ₃₁ , R ₃₂ , R ₃₃ , R ₃₄ : n-butyl group compound; R ₃₁ , R ₃₂ , R ₃₃ , R ₃₄ : n- Examples thereof include compounds having an octyl group.

Next, the general formula (IV) will be described in detail.

[Chemical 12]

In the general formula (IV), R ₄₁ , R ₄₂ , R
₄₃ and R _{44, which} may be the same or different, each represents a linear or branched saturated alkyl group, and has the same meaning as R ₁ in formula (I).

Z ₁ and Z ₂ may be the same or different and each is a halogen atom, a hydroxy group, an acyloxy group (-OCOR) derived from a fatty acid (RCOOH) having up to 18 carbon atoms, or a thiocyanate group (- NCS), an alkylsulfonyloxy group or an arylsulfonyloxy group. Acyloxy group (-OC) derived from halogen atom and fatty acid (RCOOH) having up to 18 carbon atoms
OR), an alkylsulfonyloxy group, and an arylsulfonyloxy group, X _{1 to} X in the general formula (I)
_{The same as 3} is mentioned.

The compound of the general formula (IV) may form the following dimer (1,3-disubstituted tetraorganodistannoxane).

[0039]

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Preferred compounds of general formula (IV) are R
₄₁ , R ₄₂ , R ₄₃ , R ₄₄ : n-butyl group, Z ₁ , Z
₂ : Acetoxy group compound; R ₄₁ , R ₄₂ ,
_{R 43,} R _44: n- butyl _group, Z _1, Z 2: -OCOC
₁₁ H ₂₃ compounds and the like can be mentioned.

Preferred dimeric compounds of formula (IV) are R ₄₁ , R ₄₂ , R ₄₃ , R ₄₄ : n-
Butyl _group, Z _1, Z 2: compound of -NCS group;
R ₄₁ , R ₄₂ , R ₄₃ , R ₄₄ : n-butyl group, Z ₁ :
-NCS group, _{Z 2:} a compound of a hydroxy group; _{R 41,}
R ₄₂ , R ₄₃ , R ₄₄ : n-butyl group, Z ₁ , Z ₂ : C
Compound of l; R ₄₁ , R ₄₂ , R ₄₃ , R ₄₄ : n-butyl group, Z ₁ : Cl, Z ₂ : compound of hydroxy group;
R ₄₁ , R ₄₂ , R ₄₃ , R ₄₄ : methyl group, Z ₁ , Z
₂ : A compound of -NCS group and the like can be mentioned.

The compounds of the general formulas (I) to (III) can be obtained, for example, by the following reaction.

In general, tetraalkyltin and tetraphenyltin can be synthesized by reacting trialkylaluminum, alkyl chloride and alkyl magnesium chloride corresponding to tin chloride (tetravalent) (Inaba, Organic Synthetic Chemistry, 23, 806 (1965)). In the formulas below, R ′ and R ″ represent an alkyl group.

[0044]

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The trihalogenated monoalkyltin can be synthesized by reacting tetraalkyltin with tin chloride.

[0046]

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The monoalkyltin trihalogenide can be further converted into a monoalkyltin derivative.

[0048]

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The compound of the general formula (IV) can be synthesized, for example, as follows. That is, acetic anhydride (1 equivalent) was added to dibutyltin oxide (2 equivalents) in an inert solvent (toluene, xylene, hexane, etc.) at 50-60.
The desired product can be obtained in good yield by heating at ℃ for 1-2 hours and distilling off the solvent.

[0050]

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When the compound of the general formula (IV) is a dimer, it can be synthesized by partial hydrolysis of diorganotin dihalide in the presence of an alkali (Okaw
ara, R .; Wada, M. Adv. Organomet. Chem. 1967, 5, 13
7).

Next, the lactide production method of the present invention will be described. In the method of the present invention, the general formula (I)
In the presence of at least one of the lactide-forming catalysts represented by (II), general formula (III) or general formula (IV), the lactic acid oligomer is heated under reduced pressure to be depolymerized and converted into lactide.

The lactic acid oligomer used in the method of the present invention is
From the viewpoint of preventing lactic acid or a chain dimer of lactic acid from being mixed in the lactide distillate at the time of depolymerization, the degree of polymerization is generally about 8 to 25 (molecular weight of about 600 to 2000), but this range It is not limited to. Such a lactic acid oligomer reduces the lactic acid monomer under reduced pressure (generally 15
Heating under 10 mmHg) (generally 120-160)
C.).

In the present invention, the isolated lactic acid oligomer may be used, but the lactic acid oligomer obtained by heating the lactic acid monomer under reduced pressure may be subjected to the depolymerization reaction without isolation.

In the depolymerization reaction, the above general formulas (I) to (I
The addition amount of the lactide-forming catalyst represented by either V) is 0.0 of the total lactic acid units constituting the lactic acid oligomer to be treated.
It is preferably from 1 to 1 mol%. If it is less than 0.01 mol%, the action as a catalyst is insufficient, while if it is added in excess of 1 mol%, the catalytic action does not change, and it is not economically preferable. In the case of the dimer represented by the general formula (IV), the addition amount of the monomer may be the above amount.

The heating temperature in the depolymerization reaction is generally 130 to 230 ° C. under reduced pressure, preferably 190 ° C. or lower. More preferable heating temperature is under reduced pressure (3
It is 180 to 190 ° C. in mmHg). If it exceeds 190 ° C., the optical purity of the lactide obtained tends to decrease.

That is, according to the depolymerization reaction of the lactic acid oligomer represented by the following formula, the optical purity of the lactide formed is determined according to the absolute configuration of the asymmetric carbon atom of the lactic acid unit constituting the lactic acid oligomer.

[0058]

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However, it is considered that when the heating temperature in the depolymerization reaction is high, the steric inversion of the asymmetric carbon atom occurs and the optical purity of the produced lactide is lowered. By using the depolymerization catalyst of the present invention, a lactic acid oligomer can be depolymerized at a relatively low temperature.
From the oligomer of L, L having an optical purity of 99.5% ee or more
-Lactide can be obtained.

The degree of reduced pressure in the depolymerization reaction is a pressure equal to or lower than the vapor pressure of lactide at the heating temperature, and is usually 1
It is about 100 mmHg. A lower pressure is preferable because the heating temperature can be lowered, and therefore 1 to
10 mmHg is preferable, and 1-5 mmHg is more preferable.

By controlling the temperature and the degree of reduced pressure as described above, the lactic acid oligomer melts and becomes uniform with the catalyst, the depolymerization reaction proceeds, and the lactide formed can be distilled off. For example, under a reduced pressure of 2 mmHg, 90 to 115
Lactide is obtained as a fraction at ° C. The time required for the depolymerization reaction is usually about 2.5 to 3.5 hours.

According to the present invention, since a tin compound having a specific structure is used as a depolymerization catalyst for a lactic acid oligomer, the lactic acid oligomer can be depolymerized at a relatively low temperature to produce a lactide having a high chemical yield and a high optical purity. it can.

The L, L-lactide produced according to the present invention has a high optical purity, and the poly-L-lactic acid obtained by ring-opening polymerization of this lactide has high crystallinity, and molding of the polymer is rather difficult. However, in the ring-opening polymerization, an appropriate amount of D, D-
The crystallinity of polylactic acid can be easily controlled by using lactide or meso D, L-lactide.

[0064]

EXAMPLES The present invention will be described in more detail below with reference to examples.

(GC analysis conditions of lactide produced) Column: CHROMPACK capillary column CP-Cyc
lodextrine-β-236M-19 (225-250 ° C) 0.25 mmI. D × 50 m df = 0.25 μm Shimadzu GC 9AM gas chromatography H ₂ : 0.5 Kg / cm ² Carrier gas He: 3.0 Kg / cm ² , 50 ml / min
(Make up gas) Air: 0.8Kg / cm ² Column temperature: 150 ℃ Injector temperature: 250 ℃ Detector: FID

Example 1 90.1 was added to a 1 liter distillation flask.
~ 90.2% L-lactic acid 300 g (3 mol, L-form 98.4
~ 98.6%, D-form 1.6 ~ 1.4%, PURA
C) and put it under reduced pressure (10 mmHg), 120-1.
The mixture was stirred at 50 ° C. for 7 hours to give a lactic acid oligomer. The molecular weight of the lactic acid oligomer was about 1000.

The catalyst O = Sn (C ₄ H ₉ ) (OH)
(Product name: FASCAT 4100, ELF ATOCHEM No
rth America's) 0.03 mol (1 mol% of all lactic acid units constituting the lactic acid oligomer) was added, and the pressure was reduced (2 mm).
Hg) was distilled, and the lactide fraction with bp = 90 to 115 ° C. was collected while keeping the temperature in the flask at 190 ° C. or lower. The collection time was about 3 hours. Lactide fraction 19
3 g (chemical yield of lactide: 89% based on the starting L-lactic acid) were obtained.

The lactide fraction thus obtained was subjected to GC analysis using a β-cyclodextrin-supported capillary column as described above to separate and quantify the products. LL-lactide, DD-lactide, LD-
Table 1 shows the production ratio (GC area%) of lactide (meso) and by-products. The optical purity of LL-lactide was calculated to be 1
It was 00% ee.

Example 2 In Example 1, Sn (C ₄ H ₉ ) (OH) ₂ Cl (trade name: FASC) was used as a catalyst.
AT 4101, ELF ATOCHEM North America)
The same operation as in Example 1 was carried out except that 0.03 mol was used to obtain 182 g (84%) of a lactide fraction.
The obtained lactide fraction was subjected to GC analysis to separate and quantify the product. The optical purity of LL-lactide is 9
It was 9.6% ee.

Example 3 In Example 1, Sn (C ₄ H ₉ ) Cl ₃ (trade name: CERTINCO) was used as a catalyst.
The same operation as in Example 1 was carried out except that 0.03 mol of ATTC-100, manufactured by ELF ATOCHEM North America) was used to obtain 183 g (85%) of a lactide fraction. The obtained lactide fraction was subjected to GC analysis to separate and quantify the product. The optical purity of LL-lactide was 100% ee.

Example 4 The same operation as in Example 1 was carried out except that 0.03 mol of Sn (C ₅ H ₆ ) ₄ was used as the catalyst in Example 1, and the lactide fraction 149 was obtained.
g (69%) was obtained. Regarding the obtained lactide fraction,
GC analysis was performed to separate and quantify the product. The optical purity of LL-lactide was 99.5% ee.

[0072] In Example 5 Example 1, as a catalyst _{O- [Sn (C 4 H 9} ) 2 (OCOCH 3)] 2 ( trade name: TK-1, manufactured by Takeda Pharmaceutical) 0.03 mol Except for using it, the same operation as in Example 1 was carried out to obtain 185 g (86%) of a lactide fraction. The obtained lactide fraction was subjected to GC analysis to separate and quantify the product.
The optical purity of LL-lactide was 99.7% ee.

The above results are summarized in Table 1. From Table 1, it is clear that when the tin compound of the present invention having a specific structure is used as a depolymerization catalyst, lactide with high chemical yield and high optical purity can be produced.

[0074]

As described above, the lactide-forming catalyst of the present invention is a tin compound having a specific structure and has an excellent depolymerization action of lactic acid oligomer. According to the lactide production method of the present invention using this catalyst, a lactic acid oligomer can be depolymerized at a relatively low temperature, and a lactide having a high chemical yield and a high optical purity can be produced.

[0075]

[Table 1]

Claims (3)

[Claims] 1. The following general formula (I), general formula (II) and general formula (I
II) or a lactic acid oligomer lactide-forming catalyst represented by the general formula (IV). Embedded image (In the general formula (I), R ₁ represents a saturated alkyl group, X ₁ , X ₂ , and X ₃ may be the same or different and are derived from a halogen atom, a hydroxy group, or a fatty acid having up to 18 carbon atoms. Represents an acyloxy group, an alkylsulfonyloxy group or an arylsulfonyloxy group.) (In the general formula (II), R ₂ represents a saturated alkyl group, Y represents a halogen atom, a hydroxy group, an acyloxy group derived from a fatty acid having up to 18 carbon atoms, an alkylsulfonyloxy group or an arylsulfonyloxy group. .) [Chemical Formula 3] (In the general formula (III), R ₃₁ , R ₃₂ , R ₃₃ , R
₃₄ may be the same or different and each represents a saturated alkyl group or an aryl group. ) [Chemical 4] (In the general formula (IV), R ₄₁ , R ₄₂ , R ₄₃ , R
₄₄ may be the same or different and represent a saturated alkyl group, Z ₁ and Z ₂ may be the same or different, and a halogen atom, a hydroxy group, an acyloxy group derived from a fatty acid having up to 18 carbon atoms , A thiocyanate (-NCS) group, an alkylsulfonyloxy group or an arylsulfonyloxy group. Further, the compound of the general formula (IV) may form a dimer. ) 2. A method for producing a lactide by heating a lactic acid oligomer under reduced pressure in the presence of a catalyst to produce lactide, wherein at least one of the lactide-forming catalysts according to claim 1 is used as the catalyst. A method for producing lactide, which comprises: 3. The method for producing lactide according to claim 2, wherein the amount of the catalyst used is 0.01 to 1 mol% of all lactic acid units constituting the lactic acid oligomer to be treated.