Stereodivergent Syntheses of Highly Substituted Enantiopure 4‐Alkoxy‐3,6‐dihydro‐2H‐1,2‐oxazines by Addition of Lithiated Alkoxyallenes to Carbohydrate‐Derived Aldonitrones

Abstract

Additions of lithiated alkoxyallenes to D-glyceraldehydebased nitrones 1 and 2 did not provide the expected hydroxylamine derivatives. Instead, a novel [3+3] cyclization process furnished 4-alkoxy-3,6-dihydro-2H-1,2-oxazines 9-14 with excellent syn selectivities and in moderate to good yields. Through precomplexation of the nitrones the corresponding anti-configured 1,2-oxazines 9, 10 and 13 could be obtained with high stereoselectivity. The reactions of nitrones 3-6, derived from D-erythrose or D-threose, generally proceeded less diastereoselectively, but reasonable yields of anti-configured 1,2-oxazines such as anti-17 and anti-19 could be obtained under Lewis acid promotion conditions. This was also the case for reactions of the D-arabinose-derived nitrone 7, which provided the anti-1,2-oxazines 23 and

Asymmeric Formal [3 + 3]-Cycloaddition Reactions of Nitrones with Electrophilic Vinylcarbene Intermediates

“…Greater attention has been given to azomethine imines and their reactions with trimethylenemethane (Pd catalysis), 4 enals ( N -heterocyclic carbene catalysis), 5 and propargyl esters (Au catalysis); 6 and other pairings for [3+3] cycloaddition show broad applicability of this annulation transformation. 7 However, although there is recognized high potential for [3+3]-cycloaddition between 1,3-dipoles and electrophilic vinylcarbenes, only the recent communication by Toste has described a [3+3]-annulation process that involves a putative vinylcarbene intermediate. 6 …”

An Improved Ring‐Closing Metathesis Approach to Fluorinated and Trifluoromethylated Nitrogen Heterocycles

“…Thus, benzyloxyallene 40 [18] was treated with sulfonamide 31 in the presence of Et 3 N, catalytic Pd(OAc) 2 and dppp in acetonitrile to provide N,O-acetal 41 in excellent yield. The RCM precursor 41 was subjected to the previously described RCM conditions using 7 mol-% of catalyst 10 in toluene at 100°C.…”

Crystal structure of (3R,6S,4'S)-2-benzyl-3-(2',2'-dimethyl[1',3']dioxolan- 4'-yl)-4-methoxy-6-phenyl-3,6-dihydro-2H-1,2-oxazine, C23H27NO4

“…This result can be explained by an addition of (aS)-configured 1-lithio-1-methoxy-3-phenylallene to the Si-face of the nitrone [2]. Bond distances and bond angles show normal values.…”