1,3-Dioxa-[3,3]-sigmatropic Oxo-Rearrangement of Substituted Allylic Carbamates: Scope and Mechanistic Studies

Abstract

An unexpected 1,3-dioxa-[3,3]-sigmatropic rearrangement during the treatment of aryl-and alkenyl-substituted allylic alcohols with activated isocyanates is reported. The reorganization of bonds is highly dependent on the electron density of the aromatic ring and the nature of isocyanate used. This metal-free tandem reaction from branched allyl alcohols initiated by a carbamoylation reaction and followed by a sigmatropic rearrangement thus offers a new access to (E)-cinnamyl and conjugated (E,E)-diene carbamates, such as N-acyl and N-sulfonyl derivatives. A computational study was conducted in order to rationalize this phenomenon, and a rearrangement progress kinetic analysis was performed.

Microwave‐Assisted 1,3‐Dioxa‐[3,3]‐Sigmatropic Rearrangement of Substituted Allylic Carbamates: Application to the Synthesis of Novel 1,3‐Oxazine‐2,4‐dione Derivatives

“…1.2 kcal/mol less energetic than ( Z )‐ TS1 . Both transition structures are quite synchronous and polar, with a negatively charged carboxylate moiety and a partially positively charged allyl cation, in line with previous results [12] . This polar character should favor the progress of the reaction under microwave irradiation [22] .…”

“…The first step was the carbamoylation of 1 a with 2,2,2‐trichloroacetyl isocyanate (TCA‐NCO) in toluene leading to the formation of the branched carbamate 2 a in a quantitative way monitored by 1 H NMR. In a previous paper, we showed that when the mixture reaction was undergone under conventional thermal conditions (90 °C, 20 h), the linear primary carbamate 3 a was isolated in a 50 % overall yield from 1 a after hydrolysis of the rearranged intermediate [12] . Under microwave conditions (110 °C), we were pleased to observe a complete stereoselective rearrangement only after 30 minutes.…”

“…We have also investigated the origins of the ( Z )‐selectivity in TS1 and TS2 by means of a modified distortion‐strain model based on the partitioning of the transition structure in fragments (Fragment Distortion Model) [23] . According to this modification, previously reported by our group, [12] we described the activation energy as the contribution of the deformation (FDEF) and interaction (FINT) terms between the carboxylate (C) and allylic (A) componnets of the TS under study:…”

“…( E )‐3‐(4‐chlorophenyl)allyl carbamate 3 a : [12] Prepared from 1 a and the desired product was obtained as a white solid (38 mg) in 65 % yield without any need for a purification. Mp=144–146 °C.…”

“…Recently, we brought to light an unexpected metal‐free 1,3‐dioxa‐[3,3]‐sigmatropic rearrangement of allylic carbamates from α‐aryl allylic alcohols (Scheme 1a 1 ) [12] . The first step is a carbamoylation reaction followed, in some cases, by a spontaneous rearrangement to afford the corresponding linear carbamates.…”