Stereoselective synthesis of spirocyclic cyclopentapyrans by the Pauson–Khand reaction on camphor tethered enynes

An intramolecular Pauson–Khand reaction of camphor tethered enynes derived from homoallyl, homomethallyl, and homopropargyl alcohols is described. Homoallyl, homomethallyl, and homopropargyl moieties are easily constructed on the camphor carbonyl group with excellent diastereoselectivity due to endo-face selectivity, and with known stereochemistry. Each enantiomerically pure enyne affords the conformationally most stable diastereomeric spirocyclic cyclopenta[c]pyran ring system.

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(1R,2R)-2-Allyl-1,7,7-trimethyl-2-(prop-2-ynyloxy)bicyclo[2.2.1]heptaneC16H24O[α]D25=+3.56 (c 1.7, CH2Cl2)Absolute configuration: (1R,2R)Source of chirality: (1R)-(+)-camphor

(1R,2S)-1,7,7-Trimethyl-2-(2-methylallyloxy)-2-(prop-2-ynyl)bicyclo[2.2.1]heptaneC17H26O[α]D26=+22.8 (c 1.0, CH2Cl2)Absolute configuration: (1R,2S)Source of chirality: (1R)-(+)-camphor

(1R,2S)-1,7,7-Trimethyl-2-(2-methylallyloxy)-2-(prop-2-ynyl)bicyclo[2.2.1]heptaneC17H26O[α]D26=-19.02 (c 1.0, CH2Cl2)Absolute configuration: (1R,2S)Source of chirality: (1R)-(+)-camphor

(1R,2S)-2-(Allyloxy)-1,7,7-trimethyl-2-(prop-2-ynyl)bicyclo[2.2.1]heptaneC16H24O[α]D25=-16.1 (c 1.6, CH2Cl2)Absolute configuration: (1R,2S)Source of chirality: (1R)-(+)-camphor

(1R,2R,4a′R)-1,7,7-Trimethyl-4a′,5′-dihydro-1′H-spiro[bicyclo[2.2.1]heptane-2,3′-cyclopenta[c]pyran]-6′(4′H)-oneC17H24O2[α]D25=+22.9 (c 0.75, CH2Cl2)Absolute configuration: (1R,2R, 4aR)Source of chirality: (1R)-(+)-camphor

(1R,2S,4a′R)-1,4a′,7,7-Tetramethyl-4a′,5′-dihydro-1′H-spiro[bicyclo[2.2.1]heptane-2,3′-cyclopenta[c]pyran]-6′(4′H)-oneC18H26O2[α]D20=-177.4 (c 1, CHCl3)Absolute configuration: (1R,2S,4a′R)Source of chirality: (1R)-(+)-camphor

(1R,2S,7a′R)-1,7,7,7a′-Tetramethyl-7′,7a′-dihydro-1′H-spiro[bicyclo[2.2.1]heptane-2,3′-cyclopenta[c]pyran]-6′(4′H)-oneC18H26O2[α]D20=+93.30 (c 1, CHCl3)Absolute configuration: (1R,2S,7a′R)Source of chirality: (1R)-(+)-camphor

(1R,2S,7a′R)-1,7,7-Trimethyl-7′,7a′-dihydro-1′H-spiro[bicyclo[2.2.1]heptane-2,3′-cyclopenta[c]pyran]-6′(4′H)-oneC17H24O2[α]D20=+222.6 (c 0.68, CH2Cl2)Absolute configuration: (1R,2S,7a′R)Source of chirality: (1R)-(+)-camphor