Total synthesis of the marine toxin phorboxazole A using palladium(II)-mediated intramolecular alkoxycarbonylation for tetrahydropyran synthesis.
The potent antitumor agent phorboxazole A was synthesized from six subunits comprising C1-C2 (115), C3-C8 (98), C9-C19 (74), C20-C32 (52), C33-C41 (84) and C42-C46 (85). Tetrahydropyrans B and C containing cis-2,6-disubstitution were fabricated via palladium(II)-mediated intramolecular alkoxycarbonylation which, in the case of tetrahydropyran C, was carried out with catalytic palladium(II) and p-benzoquinone as the stoichiometric re-oxidant. Tetrahydropyran D was obtained by a stereoselective tin(IV)-catalyzed coupling of a C9 aldehyde with an allylsilane, and the C19-C20 connection was made using a completely stereoselective Wittig-Schlosser (E) olefination. Coupling of the oxazole C32 methyl substituent with the intact C33-C46 δ-lactone 3was accompanied by elimination of the vinyl bromide to a terminal alkyne, but the C32-C33 linkage was implemented successfully with 83 and C33-C41 lactone 84. The C42-C46 segment of the side chain was then appended via Julia-Kocienski olefination. The macrolide portion of phorboxazole A was completed by means of an Ando-Still-Gennari intramolecular (Z)-selective olefination at C2-C3 which required placement of a (dimethoxyphosphinyl)acetate moiety at C24. Final deprotection led to phorboxazole A via a route in which the longest linear sequence is 37 steps and the overall yield is 0.36%.
Department of Chemistry, Oregon State University, Corvallis, Oregon, USA.
Punlop
P
Kuntiyong
Tae Hee
TH
Lee
Christian L
CL
Kranemann
James D
JD
White
D006576:Heterocyclic Compounds, 4 or More Rings / Q000138:chemical synthesis* / Q000737:chemistry; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D010080:Oxazoles / Q000138:chemical synthesis* / Q000737:chemistry; D010165:Palladium / Q000737:chemistry*; D011714:Pyrans / Q000138:chemical synthesis* / Q000737:chemistry
D006576:Heterocyclic Compounds, 4 or More Rings; D009942:Organometallic Compounds; D010080:Oxazoles; D011714:Pyrans; C408887:phorboxazole A; D010165:Palladium