Chiral Boron Enolate Aldol Additions to Chiral Aldehydes

We have examined the double-diastereodifferentiating aldol addition reactions of chiral enolborinate 1a with chiral aldehydes leading to the corresponding aldol adducts with excellent levels of 1,5-anti diastereoselection.

Concerning the Application of the 1H NMR ABX Analysis for Assignment of Stereochemistry to Aldols Deriving from Aldehydes Lacking β-Branches

[reactions: see text] Attempts to apply the 1H NMR ABX method for assignment of stereochemistry of beta-hydroxy ketones to aldols 4-10 deriving from alpha-methyl aldehydes lacking beta-branches reveals that the presence of a beta-branch in the aldehyde reaction partner is necessary so that the average chemical environment of Ha and Hb is different for the Felkin and anti-Felkin aldols (see conformational pairs A/B and C/D, respectively). When the chiral alpha-methyl aldehyde lacks a beta-branch, as in the case of the aldehyde precursors to 4-10, the conformational energies of E and F (for the Felkin beta-hydroxy ketone derivatives), and conformers G and H for the anti-Felkin aldols, are too close in energy (within each pair), such that the average chemical and magnetic environments of Ha and Hb in the two diastereomers cannot be easily distinguished. This analysis provides a rational basis for application of the 1H NMR ABX pattern analysis to other beta-hydroxy ketone derivatives.

Orbital Interactions and Their Effects on 13C NMR Chemical Shifts for 4,6-Disubstituted-2,2-dimethyl-1,3-dioxanes. A Theoretical Study

A theoretical study is employed to describe the orbital interactions involved in the conformers' stability, the energies for the stereoelectronic interactions, and the corresponding effects of these interactions on the molecular structure (bond lengths) for cis- and trans-4,6-disubstituted-2,2-dimethyl-1,3-dioxanes. For cis-4,6-disubstituted-2,2-dimethyl-1,3-dioxanes, two LPO --> sigma*C(2)-Me(8) interactions are extremely important and the energies involved in these interactions are in the range 6.81-7.58 kcal mol(-1) for the LP(O)(1) --> sigma*C(2)-Me(8) and 7.58-7.71 kcal mol(-1) for the LP(O)(3) --> sigma*C(2)-Me(8) interaction. These two LP(O) --> sigma*C(2)-Me(8) interactions cause an upfield shift, indicating an increased shielding (increased electron density) of the ketal carbon C(2) as well as the axial Me(8) group in the chair conformation. These LP(O) --> sigma*C(2)-Me(8) hyperconjugative anomeric type interactions can explain the 13C NMR chemical shifts at 19 ppm for the axial methyl group "Me(8)" and 98.5 ppm for the ketal carbon "C(2)". The observed results for the trans derivatives showed that for compounds 2a-c (R = -CN, -C[triple bond]CH, and -CHO, respectively) the chair conformation is predominant, whereas for 2d,f-h [-CH3, -Ph, -C6H4(p-NO2), -C6H4(p-OCH3), respectively] the twist-boat is the most stable compound and for 2e [-C(CH3)3] is the only form.

Total Synthesis of the Potent Antitumor Polyketide (−)-Callystatin A

A highly convergent and efficient total synthesis of the potent antitumor polyketide (-)-callystatin A is described. The synthesis required 19 steps from N-propionyl oxazolidinone 23 and produced the desired product in 3.5% overall yield.

Total Synthesis of (+)-Crocacin D

[structure: see text] The total synthesis of (+)-crocacin D is described. The convergent asymmetric synthesis relies on the use of a Stille cross-coupling between an (E)-vinyl stannane with an (E)-vinyl iodide to establish the (E,E)-dienamide moiety followed by a mild and efficient copper-catalyzed coupling between (+)-crocacin C and a (Z)-vinyl iodide to establish the challenging (Z)-enamide function.

Short Synthesis of the 6,6-Spiroketal Cores of Spirofungins A and B

[reaction: see text] Initial efforts toward the total synthesis of the antifungal antibiotics spirofungins A and B are reported. A short and efficient synthesis of the C9-C20 6,6-spiroketal fragments of both compounds is described. This asymmetric approach uses a very efficient alkylation of a lithiated N,N-dimethylhydrazone followed by spiroketal formation under acidic conditions.

Total synthesis of (-)-pironetin

[structure: see text] The asymmetric total synthesis of pironetin, a compound that shows plant growth regulatory activity, immunosuppressive as well as a remarkable antitumoral activity, is described. The approach involves the use of three very efficient Evans oxazolidinone-mediated syn-aldol condensations, a high-yielding coupling between lithium acetylide ethylenediamine complex and a tosylate followed by methylation, and selective reduction to establish the C12-C13 (E) double bond.

High 1,5-anti stereoinduction in boron-mediated aldol reactions of methyl ketones

[reaction: see text] We report herein a very efficient and synthetically useful 1,4-anti-1,5-anti boron-mediated aldol reaction of a chiral alpha-methyl-beta-alkoxy methyl ketone with achiral aldehydes.