Synthesis of Vinyl- and Alkynylcyclopentanetetraols by SmI₂/Pd(0)-Promoted Carbohydrate Ring-Contraction

Abstract

A variety of vinyl- or alkynyl-substituted polyhydroxylated cyclopentanes and cyclobutanes are prepared in enantiomerically pure form from appropriate carbohydrate precursors, in a direct one-step ring-contraction procedure promoted by SmI2 and catalytic Pd(O). This reaction is thought to proceed through intermediate ring-opened allyl- or allenylsamarium complexes that undergo ring-closure by intramolecular carbonyl addition. A predominant trans relationship is found between vinyl (or alkynyl) and hydroxyl groups at the two newly created stereogenic centers, with good to excellent levels of stereoselectivity being observed in the formation of homopropargyl cyclopentanol products. Under appropriate conditions, preparatively useful yields are realized of stereoisomers not directly available using alternative methodology.

Ring contraction versus β-elimination in reactions of alkynyl-substituted bicyclic lactol esters with SmI2/Pd(0)

“…10 In monocyclic systems with terminal alkynes the oxidative addition is reasonably fast and, as a result, ring opening is favored over β-elimination. 5,6 This also seems to be the case with cyclopentanone-derived substrates 1d and 4a. If, on the other hand, the oxidative addition step becomes slower due to the presence of terminal alkynyl substituents, then β-elimination takes over as it is observed in the reactions of 1e and 4b.…”

“…Carbohydrate-derived monocyclic acetals and esters related to 1, 2 and 4 have been employed in ring raction reactions leading to 2-alkynylcyclopentanols, 5,6 a transformation that is readily understood in the mechanistic terms shown in Scheme 1. This reaction is facilitated by Lewis acidic Sm(III) species, inevitably present in the reaction medium, that activate the leaving group (OMe or OCOR) and bring about the initial oxidative addition step that begins the catalytic cycle.…”

Stereoselective Glycal Fluorophosphorylation: Synthesis of ADP‐2‐fluoroheptose, an Inhibitor of the LPS Biosynthesis

“…OsO 4 (2.5 % in tBuOH, 1.82 mL) was added to a solution of olefin 4 [36] (2.18 g, 4.72 mmol) and NMO (1.27 g, 9.44 mmol) in acetone (25 mL) and water (3 mL). After 13 h at room temperature, the reaction mixture was poured into aq.…”

Zirconocene-mediated deoxygenative ring contractions of vinyl-substituted carbohydrates: An expedient route to enantiomerically pure, densely functionalized 4-to-8-membered carbocycles

“…This tendency to experience anomerization suggests that 37 is possibly generated and that advancement to 38 may even occur. Further progress toward 40 is precluded because of the cis orientation of the three OR substituents on the periphery of the nine-membered ring in 39 Contrariwise, the OR groups in glycoside 42 do not share in the same level of steric congestion as is present in 36 with the result that 43, once formed, finds it possible to produce 46 via 44 and 45. In agreement with expectation, the cyclic nature of the chair-like transition state represented by 45 delivers 46 with excellent diastereoplacement of the cisoriented vinyl and hydroxyl substituents.…”