Highly Stereoselective Synthesis of anti,anti-Dipropionate Stereotriads: A Solution to the Long-Standing Problem of Challenging Mismatched Double Asymmetric Crotylboration Reactions

The stereocontrolled synthesis of the beta-branched anti,anti-dipropionate stereotriad 4 via aldol or crotylmetal chemistry represents a historical challenge to the organic synthesis community. Here we describe a general solution to the long-standing problem associated with the synthesis of 4 by utilizing mismatched double asymmetric crotylboration reactions of enantioenriched alpha-methyl substituted aldehydes with the chiral, nonracemic crotylborane reagent (S)-(E)-22 (or its enantiomer). This method not only provides direct access to anti,anti-dipropionate stereotriads 24 [a synthetic equivalent of 4] with very good (5-8:1) if not excellent (>= 15:1) diastereoselectivity from beta-branched chiral aldehydes with <= 50:1 intrinsic diastereofacial selectivity preferences but also provides a vinylstannane unit in the products that is properly functionalized for use in subsequent C-C bond-forming events. We anticipate that this method will be widely applicable and will lead to substantial simplification of strategies for synthesis of polyketide natural products.