Epoxides vinylic, reaction with Grignard reagents

Allylic halides couple effectively with vinyltin reagents to provide the corresponding 1,4-dienes in high yields.2 Like nickel- and palladium-catalyzed Grignard reactions with allylic ethers, this reaction also proceeds with net inversion of configuration at the allylic center. Similarly, vinyl epoxides react... 

The stereospecific synthesis of (20S,22R)-17a,20,22-trihydroxycholesterol (388) and its (20S,22S)-isomer (389) has been reported. Addition of vinyl Grignard reagent to the known 16a.l7a-oxidopregnenolone acetate (385), followed by opening of the epoxide with lithium aluminium hydride, furnished the 17,20-dihydroxy-intermediate (386). Conversion into the 3,5-cyclo-steroid (387) and epoxidation of the remaining double bond afforded a C-22 epimeric mixture of epoxides which, on reaction with s-butyl-lithium and reconversion into the 3jS-hydroxy-A -steroids, afforded the epimeric tetraols (388) and (389). The preparation of similar 3,17,20-triols is also reported. ... 

The introduction of functionalized two-carbon side-chains, as found in (1) and (4) has received some attention. The usual way is by reaction of a suitable keto-derivative with a vinyl Grignard reagent, followed by functionalization of the vinyl group by way of epoxidation. In this way Fraser-Reid and Walker achieved a synthesis of pillarose (2,3,6-trideoxy-4-C-hydroxyacetyl-D- /zreo-hexose) (5) from the 4-uloside (6). As an alternative method, Brimacombe s group have published details of the use of 1-methoxyvinyl lithium, which reacts with keto-compounds to give a 1-methoxy vinyl side-chain capable of functionalization in various ways. For example, Scheme 2 shows its application to 1,2 5,6-di-0-isopropylidene-a-D-n6o-hex-3-ulose. Reaction of this reagent with methyl... 

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