Regioselective Carbon–Carbon Bond Cleavage in the Oxidation of Cyclopropenylcarbinols

Abstract

The strained double bond of cyclopropenylcarbinols undergoes a facile oxidation reaction to lead to unsaturated carbonyl derivatives. The distribution of the formed products depends on the relative stability of carbon-centered radical species, and the Sharpless kinetic resolution leads to enantiomerically pure Baylis-Hillman enal adducts.

Asymmetric Assembly of All‐Carbon Tertiary/Quaternary Nonadjacent Stereocenters through Organocatalytic Conjugate Addition of α‐Cyanoacetates to a Methacrylate Equivalent

“…The crude product was purified by flash column chromatography (pentane/Et 2 O 95:5) to obtain compound 1 . Yield: 833 mg (65 %), colorless oil; 1 H and 13 C NMR spectra were identical to those reported in the literature …”

[3,3]‐Sigmatropic Rearrangement of Cyclopropenylcarbinyl Cyanates: Access to Alkylidene(aminocyclopropane) Derivatives

“…Due to the peculiar structure and bonding properties of the cyclopropene ring, cyclopropenylcarbinols should rather be compared to propargylic rather than to allylic alcohols and it was of interest to check whether the [3,3]‐sigmatropic rearrangement of cyclopropenylcarbinyl cyanates would proceed with chirality transfer. Carbamate ( R )‐ 3 a ( ee =88 %), prepared from the optically enriched cyclopropenylcarbinol ( R )‐ 1 a , was engaged in the sequence devised previously (TFAA/Et 3 N, CH 2 Cl 2 , −78 °C, 1 h, then morpholine, −78 °C to RT) and the optically enriched urea (−)‐ 6 possessing an enantiomeric excess of 86 % was isolated (75 %) . This latter result demonstrates that a high level of chirality transfer can be achieved in the allylic transposition of cyanate 4 a (Scheme ).…”

Rhodium(II)‐Catalyzed Isomerization of Cyclopropenylmethyl Esters into (Acyloxymethylene)cyclopropanes

“…To gain insight into the degree of bond formation and bond cleavage in transition state TS1 , enantioenriched cyclopropenylmethyl acetate ( R )‐ 1 a (enantiomeric excess ( ee )=99 %) was engaged in the rhodium‐catalyzed rearrangement. The resulting methylenecyclopropane ( R )‐ 2 a ( E / Z >96:4) was obtained in 95 % ee , which demonstrated that only a slight erosion of the enantiomeric purity occurred during the rearrangement.…”