Total synthesis of mycalamide A and 7-epi-mycalamide A

[formula: see text] The final stages of a total synthesis of mycalamide A are described. A key step is the aldol reaction (mismatched) of imide 4 and aldehyde 5 which provided a ca. 5:4 mixture of aldols 10a and 10b, with incorrect C(7) stereochemistry. Elaboration of the 10a-10b mixture to mycalamide A required epimerization of C(7) at the stage of beta-keto imide 11. Alternatively, Swern oxidation of the 10a-10b mixture under conditions that minimize C(7) epimerization led to 7-epi-mycalamide A selectively.     

An NMR method for assigning relative stereochemistry to beta-hydroxy ketones deriving from aldol reactions of methyl ketones

We describe a simple 1H NMR analysis that permits the stereochemistry of beta-hydroxy ketones to be assigned by visual inspection of the ABX patterns for the alpha-methylene unit of the beta-hydroxy ketone in the 1H NMR spectra. This method has been verified by application to a wide range of beta-hydroxy ketones deriving from aldol reactions of chiral aldehydes with a variety of chiral and achiral methyl ketone enolates (see Tables 1 and 2). The stereochemistry of 54 of these compounds have been assigned by rigorous chemical methods.     

Highly stereoselective and modular syntheses of 10-hydroxytrilobacin and three diastereomers via stereodivergent [3 + 2]-annulation reactions

A convergent synthesis of the annonaceous acetogenin, 10-hydroxytrilobacin ( 4a), was accomplished by using the [3 + 2]-annulation reaction of tetrahydrofuranyl carboxaldehyde 2a and allylsilane 3. The stereodivergency of the [3 + 2]-annulation reaction made it possible to achieve modular, highly stereoselective syntheses of three 10-hydroxytrilobacin diastereomers from the same precursors by using simple modifications of reaction conditions.     

Studies on the synthesis of apoptolidin A. 2. Synthesis of the disaccharide unit

Disaccharide 3 correspoinding to the disaccharide unit of apoptolidin A has been synthesized via the regio- and stereoselective TBS-OTf-promoted beta-glycosidation reaction of 2,6-dideoxy-2-iodo-beta-glucopyranosyl acetate (5) and p-methoxybenzyl 2,6-dideoxy-2-iodo-3-C-methyl-alpha-mannopyranoside (11).     

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 β-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 α-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 β-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.     

Enantioselective synthesis of (-)-basiliskamide A

Basiliskamide A is an antifungal polyketide natural product isolated by Andersen and co-workers from a Bacillus laterosporus isolate, PNG-276. A nine-step enantioselective synthesis of (-)-basiliskamide A is reported, starting from commercially available β-hydroxy ester 7. The synthesis features a highly diastereoselective mismatched double asymmetric δ-stannylallylboration reaction of aldehyde 5 with the bifunctional allylborane reagent 4.     

Kapranov rank vs. tropical rank

We show that determining Kapranov rank of tropical matrices is not only NP-hard over any infinite field, but if solving Diophantine equations over the rational numbers is undecidable, then determining Kapranov rank over the rational numbers is also undecidable. We prove that Kapranov rank of tropical matrices is not bounded in terms of tropical rank, answering a question of Develin, Santos, and Sturmfels.

     

Thermodynamic control of 1,3-boratropic shifts of α- and γ-stannyl-substituted allylboranes: hyperconjugation outweighs steric effects

(E)-δ-Stannyl homoallylic alcohols are prepared by an allene hydroboration-aldehyde allylboration sequence ( Chen , M. et al. J. Am. Chem. Soc. 2010 , 132, 7881 ). Key to this reaction sequence is that the kinetic allene hydroboration product, 2a, is less stable than and isomerizes to the more sterically congested α-stannylallylborane 3a (see abstract figure). An M06-2X density functional analysis shows that the C-Sn to boron σ-π hyperconjugation interaction is sufficiently stabilizing to override the steric congestion in 3a.