Photoactivated Tungsten Hexacarbonyl-Catalyzed Conversion of Alkynols to Glycals

Abstract

The photoactivated W(CO)(6)/DABCO/THF system has been used for the formal endo-cyclization of alkynes to pyran rings. We found that the regioselectivity of ring closure depends on the relative configuration of the 3,5-dihydroxy-1-alkynes, as well as, more decisively, on the type of O-protective group. Oxygen substitution at the propargylic carbon slows the rate of alkyne insertion and allows for dihydrofuran formation through exo-cyclization. In contrast, the use of bulky silyl ethers or carbon substituents leads to dihydropyrans through endo-cyclization. Substrates bearing leaving groups such as esters, phenols, or thiophenols at the propargylic site eliminate and thus represent a limitation to the cycloisomerization methodology. Propargyl vinyl ethers will rearrange to give dienals instead of glycals. 1,2-Wittig rearrangement products of dihydropyrans are readily prepared and converted to complex bicyclic building blocks for organic synthesis.

Metal Vinylidenes as Catalytic Species in Organic Reactions

“…[31] The terminal alkyne substrate 44 was prepared by Corey-Fuchs homologation of the commercially available 3-oxopregn-4-ene-20β-carboxaldehyde (43). [32] In this transformation, the enone present in the substrate was effectively protected from side reactions by conversion to the corresponding enolate prior to the reaction.…”

“…Wipf has investigated these conditions with respect to the stereochemistry and nature of substituents on the chain linking the alkyne and the alcohol in the substrate. [43] McDonald has successfully applied these new conditions in syntheses of (L)-vancosamine (93) [44] and (D)-desosamine (95) [45] glycals (Scheme 27).…”

An oxepinone route to carbohydrate based oxepines

“…We speculated that the propargylic alcohol present in 3 was interacting with the metal catalyst in such a manner as to prevent metal vinylidene formation. 11 An alternative approach became evident during the investigation of the photocycloisomerization reaction. After attempts to selectively protect the propargylic alcohol as its benzyl or silyl ether failed, we proposed to convert it to the ketone.…”

Synthesis of Heterocycles and Carbocycles by Electrophilic Cyclization of Alkynes

“…Finally, furyl esters can be obtained from 54 Tungsten chemistry has provided a novel approach to cyclic ethers. The photocyclization of 1-alkyn-5-ols in the presence of W(CO) 6 plus DABCO affords either five-or six-membered ring unsaturated ethers depending on the substitution pattern of the alkynol (Equation 2.19) [20]. Tungsten can also effect cyclization of 4-alkyn-1-ols with incorporation of an aldehyde moiety (Equation 2.20) [21].…”