Oxidative spirocyclisation routes towards the sawaranospirolides. Synthesis of ent-sawaranospirolides C and D

Organic and Biomolecular Chemistry

Volumn 8, Issue 1, 2010, Pages 226-233

  Robertson, Jeremy ^a   Chovatia, Praful T ^a   Fowler, Thomas G ^a   Withey, Jonathan M ^a   Woollaston, Daniel J ^a  

^a UNIVERSITY OF OXFORD   (United Kingdom)

Author keywords

[No Author keywords available]

Indexed keywords

ARYLPROPANOIC ACIDS; BUTENOLIDES; DIHYDROPYRAN;

ACIDS;

SPIRO COMPOUND; SPIROLIDE A; SPIROLIDE C; SPIROLIDE D;

ARTICLE; CHAMAECYPARIS; CHEMISTRY; CYCLIZATION; OXIDATION REDUCTION REACTION; STEREOISOMERISM; SYNTHESIS;

CHAMAECYPARIS; CYCLIZATION; OXIDATION-REDUCTION; SPIRO COMPOUNDS; STEREOISOMERISM;

CHAMAECYPARIS PISIFERA;

EID: 76849106266     PISSN: 14770520     EISSN: None     Source Type: Journal    
DOI: 10.1039/b918091e     Document Type: Article

References (28)

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2. This is based on the proposed biosynthesis described in ref. 1, and references cited therein.
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4. For our recent applications of this approach in the context of natural product synthesis see: (a) J. Robertson, P. Meo, J. W. P. Dallimore, B. M. Doyle and C. Hoarau, Org. Lett., 2004, 6, 3861-3863;
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11. Although reduction with L-Selectride favoured the formation of isomer 10α, benzoyl migration complicated product isolation which resulted in a lower overall yield.
12. The stereochemistry of 10α/10β was assigned retrospectively following spirocyclisation.
13. D. Woollaston, D. Phil. Thesis, Oxford, 2007..
14. In another approach (see ref. 10) we prepared the 3,5-dibromofuryl variant of substrate 12 in order to provide a handle for introducing the p-hydroxyphenyl group by cross-coupling. However, the two bromine substituents deactivated the furan and we were unable to effect oxidation.
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23. We note that this represents the stereochemically matched case; repetition of this sequence starting with ent-31 gave a mixture of diastereomers in the conjugate addition. Thus, although the cheaper valinol-derived auxiliary serves for some purposes, the tert-leucinolanalogue (ref. 15) still remains the most generally useful.
24. This amide was examined as an oxidative cyclisation substrate but under a variety of conditions satisfactory results were not achieved; instead, recovered starting material, eliminated material, and overoxidised products were obtained.
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26. 2 (1.1 equiv.) in THF at RT gave a similar yield of spirocycles 41a, b, essentially free of the simple acid-catalysed spirocyclisation products.
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28. This coupling pattern suggests that a significant proportion of the conformers bear a 1,2-diaxial arrangement of the silyloxy substituents, which was supported by preliminary molecular mechanics calculations (Monte Carlo conformational search, MMFF).