Menthyl phenylglyoxylate

The double bonds in certain heterocyclic compounds, such as furans, Af-acylpyrroles and A-acylindoles are also susceptible to photoaddition of carbonyl compounds to form oxetanes (equation 106) (77JHC1777). A wide range of carbonyl compounds can be used, including quinones, a-diketones, acyl cyanides, perfluorinated aldehydes and ketones and esters. A remarkable case of asymmetric induction in oxetane formation has been reported from optically active menthyl phenylglyoxylate and 2,3-dimethyl-2-butene the oxetane product obtained after hydrolysis of the ester group had an optical purity of 53% (79AG(E)868). 

Diastereomeric oxetanes are formed from chiral carbonyl compounds such as menthyl phenylglyoxylate (78) (Buschmann et al., 1989). 

Oxetans.—Stable oxetens are rare another example (56), this time stabilized by the pstfluoroalkyl effect , has been reported. The Paterno-Buchi reaction cannot be used for the synthesis of 2,2-dialkyl-oxetans from aliphatic ketones. A convenient, one-step conversion of such ketones into oxetans involves use of the sulphoximide (57) and proceeds via ring-expansion of an epoxide intermediate. Unusually high asymmetric induction has been observed in the Paterno-Buchi reaction of (-)-menthyl phenylglyoxylate with tetramethylethylene. The visi-... 

Fig. 1. Application of Prolog s rule to the reduction of (—)-menthyl phenylglyoxylate. Hydrogen is the small, —CHs— the medium, and —CH(CH3)2— the large group on the asymmetric carbon.

The first report concerning an asymmetric Paterno-Buchi reaction with a chiral carbonyl component was reported in 1979 by Gotthardt and Lenz [128]. The photocycloaddition of the enantiomerically pure menthyl ester of phenylglyoxylic acid 133 with 2,3-dimethyl-2-butene gave the oxetane 134 with a diastereomeric excess of only 37% (Sch. 45). 

Cross-aldol reactions involving the optically active keto-esters menthyl pyruvate and phenylglyoxylate and either silyl enol ethers or keten silyl acetals have been shown to result in appreciable asymmetric induction, considerably larger than that observed in the reactions of the same chiral a-keto-esters with Grignard... 

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