Dedicated to Professor Rolf Huisgen on the occasion of his 85th birthday A new strategy for the stereoselective synthesis of 4-alkylidenebut-2-eno-4-lactones ( g-alkylidenebutenolides) with (Z)-configuration of the exocyclic CC bond at C(4) was developed. It is exemplified by the synthesis of 4-alkylidenebutenolactone 31 (Scheme 4), which constitutes a substructure of the carotenoids pyrrhoxanthin (1) and peridinin. The formation of the precursor 4-(1-hydroxyalkyl)butenolactone 29 was accomplished either by cyclocarbonylation of the prop-2-yn-1-ol moiety of 27 (3 29) or by hydrostannylation of the isopropylidene-protected alkynoic acid ester 26 (3 28) followed by transacetalization/transesterification (3 30). The 4-alkylidenebutenolactone was formed by the anti-selective Mitsunobu dehydration 29 3 31.
Borylation of Organo Halides and Triflates Using Tetrakis(dimethylamino)diboron. -In situ borylation of various halides and triflates renders possible the approach to boronic ester structures for subsequent Suzuki couplings. -(BELLO, C. S.; SCHMIDT-LEITHOFF*, J.; Tetrahedron Lett. 53 (2012) 46, 6230-6235, http://dx.
The regioselectivity of Sharpless asymmetric dihydroxylation (AD) of a,b,g,d-unsaturated carboxylic esters was studied as a function of a-, b-, and d-substituents and for fluorine-free versus fluorinated esters. The latter showed increased or complete g,d-selectivities: the hexafluoroisopropyl ester being superior to the trifluoroethyl ester. Olefinations of a,b-unsaturated aldehydes with phosphorus ylide 36 or phosphonate anion 41 provided a,b,g,d-unsaturated trifluoroethyl esters, leading inter alia to complete trans selectivity and to 31 with 94% E selectivity, respectively.
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