Squalene diepoxide

Compounds of oc-onocerin (onocol) type, found in some higher plants, e.g., in Ononis spinosa, probably are formed from 2,3 27,28-squalene diepoxide (Fig. 119). 

The earliest general method for the reductive deoxygenation of oxiranes to alkenes involved multistep procedures in which halohydrins were intermediates. Comforth, for example, used an oxirane deoxygenation in an early synthesis of squalene (Scheme 11). Diastereoselective double addition of ho-mogeranyllithium (23) to the dichloro diketone (24) gave the bis(chlorohydrin) (25), which was first converted to the diepoxide (26). Subsequent treatment with Nal and Zn in HOAc buffered with NaOAc gave squalene (27). 

Cyclization Reactions of Oxirans. The search for non-enzymatic cyclizations of squalene oxide and its analogues continues with the report of the direct sterol synthesis of (128) (2%) from (127). Although the yield is low, the procedure involving treatment of (127) in CH2CI2 that contains BF3 -OEt2 and ethene carbonate at 0°C for 20 minutes affords four new rings and seven new asymmetric centres in one laboratory operation. The diepoxide... 

The properties of a soluble enzyme system from plants which catalyses cyclization of squalene 2(3),22(23)-diepoxide into oc-onocerin (28) have been reported. The enzyme requires both substrate epioxide groups for recognition and cyclization of the substrate. Additional inhibitors of squalene 2,3-oxide cyclase have been described, and a comparison of the effects of numerous hypocholesterolaemic agents on squalene metabolism in rat liver and the protozoan Tetrahymena pyriformis has been reported. ... 

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