Crotylchromium reagents

The chromium-mediated addition of propargyl halides to carbonyl compounds was studied by Gore and coworkers. ° Unlike the crotylchromium reagents already described (vide supra), which react ex-... 

Kishi and co-workers elegantly demonstrated the power of substrate-directed cro-tylation reactions in their synthesis of the C(15)-C(29) segment 83 of rifamycin S (Fig. 11-10) [61]. This synthesis utilizes two crotylation reactions with the Noza-ki-Hiyama crotylchromium reagent 8 [18, 19] and one allylation reaction using allyldichloroiodostannane [79]. 

Reactions of the crotylchromium reagent (8) and a-methyl chiral aldehydes are summarized in Scheme 30." " The results with (169)-(173) demonstrate that diastereoselectivity via a transition state analogous to (140) increases as the steric demands of R increase, while the data for (174)-(177) indicate that diastereoselectivity in these more stereochemically complicated cases depends subtly on the stereochemistry of the centers at C-3 and C-4 relative to C-2. This eHect undoubtedly is related to the conformational preferences of the C(2)—C(3) bond, which influences the nonbonded interactions involving the... 

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