Preparation, Structure, and Unique Thermal [2+2], [4+2], and [3+2] Cycloaddition Reactions of 4Vinylideneoxazolidin‐2‐one

Abstract

The terminal allene C(alpha)=C(beta) bonds of 4vinylidene-2-oxazolidinone (2) readily undergo [2+2] cycloaddition with a wide variety of terminal alkynes, alkenes, and 1,3-dienes irrespective of their electronic nature under strictly thermal activation conditions (70-100 degrees C) and provide 3substituted (Z)-methylenecyclobutenes 6, 3substituted methylenecyclobutanes 7 and 8, and 3vinylmethylenecyclobutanes 9, respectively, in good to excellent yields. Alkenes react with 2 with complete retention of configuration. The [2+2] cycloaddition is concluded to proceed via a concerted [(pi(2s)+pi(2s))(allene) + pi(2s)] Hückel transition state on the basis of experimental evidences and quantum mechanical methods. Some highly polarized enones and nitrile oxide, on the other hand, react with 2 selectively at the internal C(4)=C(alpha) double bonds and give spiro compounds 10 and 11, respectively. The bent allene bonds (173-176 degrees) and the unique reactivity associated with 2 are attributed to a low-lying LUMO (C(alpha)=C(beta)) that is substantiated by a through-space sigma*(N-SO(2))-pi*(C(alpha)=C(beta)) orbital interaction.

Stereocontrolled Synthesis of Highly Functionalized Quaternary Carbon Centers: A Route to α‐Substituted Serines

“…Our initial proposal for III was allene 5 , since it can be easily obtained in two steps from but‐2‐yn‐1,4‐diol (Scheme ) 8. We reasoned that its hydroboration at the less hindered face of the terminal double bond would generate an unsymmetrical Z allylborane, which could be stereoselectively added to an aldehyde to generate the amino diols 6 that are protected at the quaternary center.…”

Stereoselective preparation of quaternary 2-vinyl sphingosines and ceramides and their effect on basal sphingolipid metabolism

“…Secondly, the N-tosyl group favors an facile acid or base-catalyzed partial isomerization to the corresponding inner N-tosylcarbamates 5 during work-up and/or chromatographic purification of compounds 4, leading to mixtures that are difficult to separate. These disadvantages can be avoided almost completely by using allene 1 (Scheme 2), obtained from but-2-yn-1,4-diol in 61% yield, 6 in which the more easily removable N-benzoyl group is used as protecting group. 7 We envisaged that the addition of allene 1 to (E)-2tridecenal would provide access to 2-vinyl sphingoid analogs 9 (Figure 1), structurally related to sphingosine.…”

Stereocontrolled Synthesis of Highly Functionalized Quaternary Carbon Centers: A Route to α‐Substituted Serines

“…Our initial proposal for III was allene 5, since it can be easily obtained in two steps from but-2-yn-1,4-diol (Scheme 2). [8] We reasoned that its hydroboration at the less hindered face of the terminal double bond would generate an unsymmetrical Z allylborane, which could be stereoselectively added to an aldehyde to generate the amino diols 6 that are protected at the quaternary center. To our delight, we found that the treatment of 5 with dicyclohexylborane in CH 2 Cl 2 at room temperature, and subsequent addition of a slight excess of isobutyraldehyde at À78 8C afforded 6 a as a single stereoisomer in 74 % yield.…”

“…[6] Examples of the stereoselective creation of quaternary carbon centers by addition of g-heteroatom allylboranes such as II are still scarce. [8] We reasoned that its hydroboration at the less hindered face of the terminal double bond would generate an unsymmetrical Z allylborane, which could be stereoselectively added to an aldehyde to generate the amino diols 6 that are protected at the quaternary center. To overcome this drawback, we anticipated that II could be prepared in a straightforward manner by hydroboration of allene III.…”

“…[8] We reasoned that its hydroboration at the less hindered face of the terminal double bond would generate an unsymmetrical Z allylborane, which could be stereoselectively added to an aldehyde to generate the amino diols 6 that are protected at the quaternary center. [8] We reasoned that its hydroboration at the less hindered face of the terminal double bond would generate an unsymmetrical Z allylborane, which could be stereoselectively added to an aldehyde to generate the amino diols 6 that are protected at the quaternary center.…”