[1,4]Dithiepino[2,3‐b]furans from Oxiranecarbaldimines and Lithiated 1,3‐Dithiane: A Series of Rearrangement Reactions in One Pot

Abstract

Oxiranecarbaldimines 1, which are easily accessible in a two-step procedure, were converted into [1,4]dithiepino [2,3-b]furans 5 by the addition of lithiated 1,3-dithiane and subsequent aqueous workup under acidic conditions. In case of bis(imine) 6, the isolation of the intermediate bis(aziridinyl alcohol) 7 was possible and resulted from an aza-Payne re-

Chiral Phosphoric Acid Catalyzed Enantioselective Ring Expansion Reaction of 1,3-Dithiane Derivatives: Case Study of the Nature of Ion-Pairing Interaction

“…These results clearly indicate that the substituents at the 3,3′-positions of BINOL-derived CPA are not essential for the specific formation of 4k and 4l in excellent diastereoselectivity. More importantly, the ion pair depicted in Scheme b (see also Scheme , Path I), which was assumed from a number of previous reports, is not suited as the plausible intermediate in this reaction. The reactions of 2k and 2l should proceed via “well-ordered” intermediates because of the stereospecific process (Scheme , Path II).…”

“…Based on our examination of previous reports, we expect that a ring expansion/deprotonation product would be formed in the absence of a nucleophile (Scheme a). The reaction of 2k or 2l in the absence of a nucleophile affords the corresponding elimination product in an enantioselective manner because of the generation of the stereogenic center at the 6-position, which reflects the chiral information originated from EDS.…”

“…The reaction is known to be initiated by a nucleophilic attack by one of the sulfur atoms of 1,3-dithiane and the elimination of the leaving group via the S N 2 mechanism. , In calculating the transition states of this initial step, the orientation of the leaving group and the location of phosphoric acid catalyst were carefully considered. The trichloroacetimidate group would adopt an axial or equatorial orientation.…”

“…To accomplish the proposed enantioselective transformation involving the thionium intermediate, we focused our attention on the 1,2-sulfur rearrangement of dithio-acetal or -ketal derivatives with a leaving group at the α-position (Scheme a) . The conventionally assumed mechanism is as follows.…”

“…On the basis of conventional assumption, a 1,3-dithiane derivative undergoes the 1,2-sulfur rearrangement via the activation of the leaving group by the CPA catalyst to generate an ion pair intermediate consisting of a chiral phosphate and an achiral thionium ion (Scheme b). It is assumed that, under the influence of chiral phosphate, the following nucleophilic addition would take place in an enantioselective manner to afford an enantio-enriched 1,4-dithiepane product.…”