[3,3]-Sigmatropic Rearrangement of Boronated Allylcyanates: A New Route to α-Aminoboronate Derivatives and Trisubstituted Tetrahydrofurans

Abstract

[3,3]-Sigmatropic cyanate-isocyanate rearrangement provides a powerful tool for the preparation of α-isocyanato allylboronic esters, which can be further trapped with a variety of nucleophiles. Hydrogenation gave the corresponding α-aminoboronates derivatives while addition of aldehydes afforded homoallylic alcohols, (tetrahydrofuran-2-yl)carbamate, ether, or urea derivatives.

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

“…As an alternative to the Overman rearrangement of trichloro‐acetimidates, the [3,3]‐sigmatropic rearrangement of allyl cyanates into the corresponding isocyanates has emerged as a powerful tool for the stereocontrolled synthesis of allylic amine derivatives . With the aim of evaluating the feasibility of the [3,3]‐sigmatropic rearrangement of cyclopropenylcarbinyl cyanates as a route toward derivatives of alkylidene (amino‐cyclopropanes), cyclopropenylcarbinol 1 a was selected as the test substrate.…”

“…As an alternative to the Overman rearrangemento ft richloroacetimidates, the [3,3]-sigmatropic rearrangement of allyl cyanates into the corresponding isocyanates hase merged as a powerful toolf or the stereocontrolled synthesis of allylic amine derivatives. [30][31][32][33] With the aim of evaluating the feasibility of the [3,3]-sigmatropic rearrangemento fc yclopropenylcarbinyl cyanates as ar oute towardd erivatives of alkylidene( amino-cyclopropanes), cyclopropenylcarbinol 1a was selected as the test substrate. Alcohol 1a was condensed with trichloroacetyl isocyanate (CH 2 Cl 2 ,08C) and the resulting N-trichloroacetyl carbamate 2a was treated under alkalinec onditions [K 2 CO 3 ,H 2 O/ MeOH (1:1),R T] [34] to afford carbamate 3a (87 %, two steps from 1a).…”

“…Alcohol 1a was condensed with trichloroacetyl isocyanate (CH 2 Cl 2 ,08C) and the resulting N-trichloroacetyl carbamate 2a was treated under alkalinec onditions [K 2 CO 3 ,H 2 O/ MeOH (1:1),R T] [34] to afford carbamate 3a (87 %, two steps from 1a). Amongt he differentr eportedp rocedures to achieve the dehydration of carbamates into cyanates and trigger their subsequent [ 3,3]-sigmatropic rearrangement, [30][31][32][33] the use of trifluoroacetic anhydride( TFAA) in the presence of triethylamine was selected. [32a] Thus, carbamate 3a was treated with TFAA (1.2 equiv) and Et 3 N( 3equiv) in CH 2 Cl 2 at À78 8Ct og enerate the corresponding cyanate 4a in situ and after 1h at À78 8C, the formation of an ew (less polar) product, tentatively assigned as isocyanate 5a,w as observed by analysis of the reaction mixture by thin layer chromatography.H owever,a ttempts to isolate this latter labile compound wereu nsuccessful.…”

[3,3]‐Sigmatropic Rearrangement/Allylboration/Cyclization Sequence: Enantioenriched Seven‐Membered‐Ring Carbamates and Ring Contraction to Pyrrolidines

“…Access to the key carbamates 1 was first achieved from optically active 1‐alkyn‐3‐ols . Dehydration with phosphine and carbon tetrabromide in the presence of triethylamine, followed by direct addition of aldehydes to the crude reaction mixture, and then aqueous NaHCO 3 15 hours later, afforded the cyclic ene carbamates 5 in good to moderate yields (three‐step process; Table ). The structure of 5 was assigned on the basis of NMR data and X‐ray crystallographic analysis of 5 a…”

The Allyl Cyanate/Isocyanate Rearrangement: An Efficient Tool for the Stereocontrolled Formation of Allylic C–N Bonds

“…α‐Aminoboronate derivatives 18 were prepared from ( E )‐boronated allylic alcohol 15 by dehydration of the carbamate 16 (Scheme ) . This step was carried out in the presence of a polymer‐supported phosphine to simplify the purification despite the longer reaction time required for complete conversion (6 h instead of 1 h at 0 °C with triphenylphosphine).…”