The
(3 + 2) cycloaddition of various indoles with a dithioallyl
cation affords dearomatized cyclopentannulated adducts, with complete
control of regioselectivity and excellent chemo- and diastereoselectivity.
The success of the reaction critically relies on the use of an excess
of very strong Brønsted acid, which paradoxically prevents carbocationic
side reactions. The reaction tolerates sensitive functionalities such
as basic amines or free hydroxyls, and we demonstrate its use in late
stage derivatization of highly functionalized, unprotected indoles.
A stereoselective gold(I)‐catalyzed vinylcyclopropanation of alkenes has been developed. A gold‐coordinated cationic vinyl carbene species, readily generated via a rearrangement of the ethylenedithioacetal of propargyl aldehyde, reacts with a wide range of alkenes to afford thio‐substituted vinylcyclopropanes. The gold‐catalyzed vinyl cyclopropanation proceeds under mild conditions at room temperature and is generally selective for the formation of cis‐substituted cyclopropanes. The reaction allows the formal introduction of a “naked” vinyl carbene, by subsequent chemoselective hydrodesulfurisation of the ethylenedithio‐bridge. The synthetic utility of the new method is demonstrated by a short, racemic formal synthesis of the alkaloid cephalotaxin, hinging on a key vinyl cyclopropane‐cyclopentene rearrangement.
This review covers the synthetic applications of 1,4-dithianes, as well as derivatives thereof at various oxidation states. The selected examples show how the specific heterocyclic reactivity can be harnessed for the controlled synthesis of carbon–carbon bonds. The reactivity is compared to and put into context with more common synthetic building blocks, such as 1,3-dithianes and (hetero)aromatic building blocks. 1,4-Dithianes have as yet not been investigated to the same extent as their well-known 1,3-dithiane counterparts, but they do offer attractive transformations that can find good use in the assembly of a wide array of complex molecular architectures, ranging from lipids and carbohydrates to various carbocyclic scaffolds. This versatility arises from the possibility to chemoselectively cleave or reduce the sulfur-heterocycle to reveal a versatile C2-synthon.
A stereoselective gold(I)‐catalyzed vinylcyclopropanation of alkenes has been developed. A gold‐coordinated cationic vinyl carbene species, readily generated via a rearrangement of the ethylenedithioacetal of propargyl aldehyde, reacts with a wide range of alkenes to afford thio‐substituted vinylcyclopropanes. The gold‐catalyzed vinyl cyclopropanation proceeds under mild conditions at room temperature and is generally selective for the formation of cis‐substituted cyclopropanes. The reaction allows the formal introduction of a “naked” vinyl carbene, by subsequent chemoselective hydrodesulfurisation of the ethylenedithio‐bridge. The synthetic utility of the new method is demonstrated by a short, racemic formal synthesis of the alkaloid cephalotaxin, hinging on a key vinyl cyclopropane‐cyclopentene rearrangement.
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