Rapid assembly of stereochemically rich polycyclic tetrahydrofurans by a conjugate addition-Rh(II) catalysis sequence

Origins of Diastereoselectivity in the Addition of Enoxysilanes to Vinyl Diazonium Salts

“…These diazo products can be used to make a variety of structurally complex substituted furans (e.g., 4). 16 We noted that in many instances the addition of enoxysilanes to vinyl diazonium ions proceeded with high levels of diastereoselectivity favoring the antiproduct (Table 1). The diastereoselectivity was greatest when the nucleophilic enoxysilane was derived from an alkyl aryl ketone that had an electron rich or electron neutral aryl or heteroaryl ring.…”

“…We recently showed that vinyl diazonium salts ( 2 , Scheme ) generated from readily available α-diazo-β-hydroxy carbonyls ( 1 ) under mild Lewis acid catalysis are strong electrophiles that will react productively at the β-position with a variety of nucleophiles. − Of these, the addition of silyl enol ethers is notable in that it is similar to a Mukaiyama-Michael addition but provides 2-diazo-1,5-diketone ( 3 ) products in high yields. These diazo products can be used to make a variety of structurally complex substituted furans (e.g., 4 ) …”

Lewis-Acid-Catalyzed Oxa-Michael Addition to Give α-Diazo-β-alkoxy Carbonyls and Tetrahydro-3H-furo[3,4-c]pyrazoles

“… , These reactions give complex scaffolds in high yields and diastereoselectivity and preserve the diazo functional group for further manipulation. For example, the 2-diazo-1,5-dicarbonyl compounds prepared by the Mukaiyama–Michael addition reaction of enoxysilanes to vinyl diazoniums can be used in subsequent carbonyl ylide 1,3-dipolar cycloaddition reactions to give 7-oxabicyclo[2.2.1]­hept-2-enes (Figure ) or in intramolecular O–H insertion reactions to give stereochemically rich monocyclic tetrahydrofurans . The genesis of that work was computational results we obtained which indicated that the Lewis-acid-catalyzed formation of vinyl diazonium ions via dehydroxylation of α-diazo-β-hydroxy carbonyls was reversible, thus opening the possibility to capture the vinyl diazonium with an alternative carbon nucleophile .…”