[reaction: see text] An enantiospecific total synthesis of Weinreb's advanced intermediate 2 for (-)-agelastatin A has been achieved from the Hough-Richardson aziridine 8. Noteworthy reactions in our sequence include the highly regioselective trans-diaxial ring-opening of 8 with azide ion to set up the vicinal diamido functionality present within (-)-2 and the Grubbs-Hoveyda ring-closing metathesis (RCM) reaction that was used to construct its cyclopentene core.

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Computed chemical shifts and coupling constants for xanthone in deuteriochloroform and trifluoroacetic acid are in agreement with experimental data at 60,100, and 220 MHz. The increased dispersion of the signals a t 220 M H z in deuteriochloroform shows four distinct aromatic proton signals which can be analysed directly by first-order methods. Fundamental parameters for xanthone in carbon tetrachloride, pentadeuteriopyridine, hexadeuterioacetone, and hexadeuteriodimethyl sulphoxide have also been measured. Additive diamagnetic shifts for urtho-, meta-, and para-hydroxy-, alkoxy-, and alkyl substituents have been determined empirically and experimental and predicted chemical-shifts for natural and synthetic xanthones are in agreement, providing steric hindrance of vicinal groups is considered. The influence of been correlated.

Four 4-n-propyl-, one 4-n-pentyl-, and four 4-phenyl-coumarins have been isolated from the bark of Marnrnea africana G. Don and identified. All are derived from 5.7-dioxygenated coumarins, and eight of the structures contain an isoprene unit and an acyl side chain a t either C-6 or C-8. In four cases the isoprene unit exists as a 3-methylbut-2-enyl side chain but otherwise it forms part of a chromen or benzofuran system. The metabolites are 5,7-dihydroxy-6-(3-methylbut-2-enyl) -8-(2-methyl butyryl) -4-n-propylcoumarin (1 ) (mammea B/B B) 5.7-dihydroxy-8-(3-methyl but-2-enyl) -6-(3-methylbutyryl) -4-phenylcoumarin (5) (mammeisin, mammea A/AA) 5.7-dihydroxy-8-(3-methylbut-2-enyl) -6-(2-rnethylbutyryl) -4-phenylcoumarin (7) (mammea A/AB, MAB 1 ), 5,7-dihydroxy-8-(3-methylbut-2-enyl)-6-(2-rnethylbutyryl)-4-n-propylcoumarin (8) (MA5 2), 4',5'-dihydro-5-hydroxy-5'-(1 -hydroxy-1 -methylethyl) -6-(2-methylbutyryl) -4-phenylfuro[2',3' :7,8]coumarin (9) (MAB 3), 4',5'-dihydro-5-hydroxy-5'-(1hydroxy-I -methylethyl) -6-(2-methylbutyryl) -4-n-propylfuro [2',3' : 7.81 coumarin (1 0) (MAB 4). 5-hydroxy-6',6'-dimethyl-6-(2-methylbutyryl)-4-phenylpyrano[2',3':7,8]coumarin(1 1) (MAB 5), 5-hydroxy-6 ',6'-dimethyl-6-(2-methylbutyryl) -4-n-propylpyrano[2',3' : 7.81 coumarin (1 2) (MAB 6) and 5,7-dihydroxy-8-(2-methylbutyryl)-4-n-pentylcoumarin (1 3). The structures of the pyranocoumarins have been confirmed by partial synthesis. The possible biogenetic relationship between the coumarins and xanthones in Guttiferae is discussed. PREVIOUSinvestigations have shown the presence of six coumarins (1)-(6) in the seeds 3a of Mammea africnna G. Don and seven xanthones have been isolated from the h e a r t ~o o d . ~ An investigation of the bark of two M . africana G. Don trees grown in Nigeria has now led to the isolation of three known coumarins [(l), (5), and (7)] (previously isolated from the seeds of M . americalza L.395) together with six new coumarins to which structures (8)-(13) are assigned. Three of