Total synthesis of (-)-spinosyn A: examination of structural features that govern the stereoselectivity of the key transannular Diels-Alder reaction.
A study of elements of stereochemical control in transannular Diels-Alder reactions leading to the decahydro-as-indacene core of (-)-spinosyn A is described. Initial studies focused on macrocyclic pentaene 9, which includes C(6)-Br and C(8)-OTBS substituents. Excellent selectivity (>95:5) was observed in the cycloaddition of 9 as a consequence of 1,3-allylic strain interactions involving the C(6) and C(8) substituents in the disfavored TS-2. The major cycloadduct 22 was used in a formal synthesis of (-)-spinosyn A. The TDA cyclizations of 12 (which lacks the C(8)-OTBS unit of 9), 13 (which lacks the C(6)-Br substituent of 12), and 14 (which lacks the C(6)-Br and C(21)-Et substituents of 12) were also studied. Macrocycles 12 and 13 served as precursors to (-)-spinosyn A and the (-)-spinosyn A aglycon (34), respectively. It is striking that substrates 12-14 give very similar distributions of transannular Diels-Alder cycloadducts, indicating that the C(6)-Br and C(21)-stereocenter do not play a significant role in the diastereoselectivity of the TDA cycloaddition of spinosyn A precursor 12. It is likely that some as yet unidentified conformational or structural features of macrocycles 12-14 contribute to the levels of diastereoselectivity achieved, since these TDA reactions are more selective for the C(7)-C(9) stereochemical relationship found in the natural product than are the IMDA reactions of trienes 4 and 7.
Winbush
SusAnn M
SM
Department of Chemistry, Scripps Florida, Jupiter, Florida 33458, USA.
Mergott
Dustin J
DJ
Roush
William R
WR
D003500:Cyclization; D018942:Macrolides / Q000138:chemical synthesis* / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D021241:Spectrometry, Mass, Electrospray Ionization; D013055:Spectrophotometry, Infrared; D013237:Stereoisomerism
D018942:Macrolides; C114883:spinosyn A