Roush asymmetric crotylation

The synthesis of the alcohol 8 started with protection of known homoallyUc alcohol 10 as its MPM ether and subsequent cleavage of the double bond to give the aldehyde 11 (Scheme 2). Roush asymmetric crotylation of 11 by using chiral crotyl boronate 12 provided the alcohol 13. Silylation of 13 followed by removal of the MPM group led to the alcohol 14, which was inverted via Mitsunobu reaction to afford the alcohol 8. Here we intentionally used the homoallylic alcohol 10 with incorrect configuration as the starting material and synthesized 8 in a circuitous manner. This was because asymmetric crotylation of ent-11, which we initially tried, only provided the alcohol 15 as an inseparable 1 1 mixture of diastereomers. 

Stevastelins are depsipeptides exhibiting immunosuppressant activity. The first total synthesis of stevastelin B was described by Y. Yamamoto and co-workers. To construct four consecutive stereocenters, the Evans aldol reaction and the Roush asymmetric allylation were utilized. In the allylation step, the authors used (S,S)-diisopropyltartrate-derived ( )-crotyl boronate. The anti homoallylic alcohol product formed as the only diastereomer. 

A total synthesis of (+)-Janomycin (121) has been reported, key steps being asymmetric crotylation of the precursor aldehyde (using Roush s tartrate boronate) generating 119, and then elaboration to the C-glycoside 120 (Scheme... 

Besides aUylsUanes or stannanes, allyl boron species have found widespread apphcations in synthesis. The Roush crotylation is a very well-estabhshed method with a broad substrate scope, and is therefore commonly used in organic synthesis [73]. The following example depicts a reversal in enantioselectivity induced by a cobalt complex present in the substrate. Roush et al. reported that the use of metal carbonyl complexes as substrate surrogates led to an improvement of enantioselectivity in the asymmetric crotylation of the respective aldehydes. These results were attributed to electronic effects exerted by the metal complexes that stabilize the transition state of the crotylation reaction (Scheme 3.47) [74]. 

Kinetic resolution can be accomplished by addition of allyl boronates to aldehyde groups adjacent to the tricarbonyliron fragment [59]. For the synthesis of ikaruga-mycin, Roush and Wada developed an impressive asymmetric crotylboration of a prochiral meso complex using a chiral diisopropyl tartrate-derived crotylborane (Scheme 1.25) [60]. In the course of this synthesis, the stereo-directing effect of the tricarbonyliron fragment has been exploited twice to introduce stereospedfically a crotyl and a vinyl fragment. 

Asymmetric allylation and crotylation, synthetically equivalent to the aldol reaction, have been extensively studied and have become a very useful procedure for preparation of propionate units. Among various chiral ligands on boron-developed, isopinocampheyl- and tartrate-derived reagents, 51 and 52, which were developed by Brown et al. [18] and Roush et al. [19], respectively, are the most commonly used (Scheme 7). Reaction of aldehyde with (Sl-Sla or 52a gave anu -adduct 54, while that using (Z)-51b or 52b afforded syn-adduct 53 with high asymmetric selectivity. 

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