Stereoselective Synthesis of Alcohols, XXVII. Addition of (α-Chlorocrotyl)boronates to Aldehydes

Catalytic Enantioselective Dihalogenation and the Selective Synthesis of (-)-Deschloromytilipin A and (-)-Danicalipin A

A titanium-based catalytic enantioselective dichlorination of simple allylic alcohols is described. This dichlorination reaction provides stereoselective access to all common dichloroalcohol building blocks used in syntheses of chlorosulfolipid natural products. An enantioselective synthesis of ent-(-)-deschloromytilipin A and a concise, eight-step synthesis of ent-(-)-danicalipin A are executed and employ the dichlorination reaction as the first step. Extension of this system to enantioselective dibromination and its use in the synthesis of pentabromide stereoarrays relevant to bromosulfolipids is reported. The described dichlorination and dibromination reactions are capable of exerting diastereocontrol in complex settings allowing X-ray crystal structure analysis of natural and unnatural diastereomers of polyhalogenated stereohexads.

Z-Selective copper-catalyzed asymmetric allylic alkylation with Grignard reagents

Allylic gem-dichlorides undergo regio- and enanantioselective (er up to 99:1) copper-catalyzed allylic alkylation with Grignard reagents affording chiral Z-vinyl chlorides. This highly versatile class of synthons can be subjected to Suzuki cross coupling affording optically active Z-alkenes and 1,3-cis,trans dienes.

Highly (E)-selective BF(3).Et(2)O-promoted allylboration of chiral nonracemic alpha-substituted allylboronates and analysis of the origin of stereocontrol

Delta-methyl-substituted homoallylic alcohols 2 were prepared in 71-88% yield, E:Z >30:1 and 93% to >95% ee via BF(3).Et(2)O-promoted allylboration with alpha-Me-allylboronate 1. The origin of high (E)-selectivity is proposed.

The origin of diastereofacial control in allylboration reactions using tartrate ester derived allylboronates: attractive interactions between the Lewis acid coordinated aldehyde carbonyl group and an ester carbonyl oxygen

Transition-state structures for the allylboration reaction between the tartrate ester and tartramide modified allylboronates and acetaldehyde are located at the B3LYP/6-31G* level of theory. An attractive interaction between the boron-activated aldehyde and the ester or amide carbonyl oxygen lone pair is found to play a major role in the favored transition states 11a and 13. This attractive interaction appears to be electrostatic in origin. However, an n --> pi* charge-transfer type of interaction has not been ruled out. The distance (2.77 A) between the aldehydic hydrogen and the carbonyl oxygen in transition state 13 is beyond the sum of van der Waals radii. The formyl C-H...O bond angle (109 degrees) in this transition structure deviates far from linearity. Therefore, hydrogen-bonding interactions between the formyl C-H and the amide carbonyl oxygen are considered negligible. The distance (3.81 A) between the aldehydic oxygen and the amide carbonyl oxygen in the diastereomeric, disfavored transition state 14 is also beyond the van der Waals radii, which suggests that n/n electronic repulsion plays a lesser role in stereodifferentiation in the allylboration reaction than originally proposed.