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Mirabimide A, the major imide from the terrestrial blue-green alga Scytonema mirabile (Dillwyn) Bornet (strain BY-8-1), is N-[2-(N,N-dimethylleucyl)oxy-3-methylpentanoyl-N-methylvalyl-prolyl]-4-methoxy-5-isopropyl-3-pyrrolin-2-one. Mirabimides B–D are the valyl, 2-oxy-3-methylbutanoyl, and N-acetyl-N-methylleucyl analogs, respectively. The absolute configurations of the five asymmetric carbons in the mirabimides are proposed to be all S , but the assignments for the dimethylleucine units in A–C and the hydroxyacid units in B–D are tentative.

Puwainaphycins A–E, one of which (C) is a potent cardioactive agent in isolated mouse atria, are cyclic decapeptides that have beeen isolated from a terrestrial blue-green alga Annabaena sp. BQ-16-1. The structures of puwainaphycins A–E, including most of the stereochemical features, have been elucidated by a combination of spectral and chemical methods.

The biosyntheses of nostocyclophane D ( 1 ) and cylindrocyclophane D ( 2 ) have been investigated by the administration of sodium [1,2- 13 C 2 ]acetate, sodium [1- 13 C, 2 H 3 ]acetate, sodium [2- 13 C, 2 H 3 ]acetate, sodium [ 2 H 3 ]acetate, sodium [1- 13 C, 18 O 2 ]acetate, sodium [2- 13 C, 18 O 2 ]acetate, l -[methyl- 13 C]methionine, and l -[methyl- 13 C, 2 H 3 ]methionine to cultures of the cyanobacteria Nostoc linckia and Cylindrospermum licheniforme , respectively. The isotopically labeled compounds produced by these feeding experiments indicate that the structural skeletons of these [7.7]paracyclophanes are polyketides that have been constructed by dimerization of acetate-derived nonaketides. Acetate has also been identified as the source of the branched methyl groups in cylindrocyclophane D.

(2S,4S)-4-Hydroxyarginine is an intermediate in the biosynthesis of anatoxin-a(s) from L-arginine.

The convergent total synthesis of cryptophycins C and D is described. It has been shown that in both natural products the absolute configuration of the a-amino acid corresponds to the D-series. The structural assignment for cryptophycin C has been corrected to reflect this fact. Since the structure of cryptophycin A has been correlated to cryptophycin C, the chloro-0-methyltyrosine unit in cryptophycin A has the D-configuration. Cryptophycins are potent tumor-selective cytotoxins associated with the terrestrial blue-green algae Nostoc sp. GSV 224' and Nostoc sp. ATCC 53789.2 The major cytotoxin in each alga, cryptophycin A, shows excellent activity against solid tumors implanted in mice, including a drug-resistant tumor. Over 20 related cytotoxins are present in the GSV 224 strain as minor constituent^,'^^ and some of these compounds, e.g., cryptophycins B and C, have been isolated in sufficient amounts for in vivo e~aluation.~ In order to acquire adequate quantities of selected naturally-occurring cryptophycins and synthetic analogs for structure-activity relationship (SAR) studies, preclinical evaluation, and human clinical trials, we have designed a general synthesis. Cryptophycins C and D, as described in the original paper, were chosen to be the initial targets as they represented examples from both of the alleged Land D-tyrosine series. We report here the total syntheses of cryptophycins C and D which (1) revise the structures of cryptophycins A and C to reflect the D-configuration for the a-amino acid unit as depicted in the structural drawings in this paper and (2) confirm the structures of cryptophycins B and D. Retrosynthetic analysis of the cryptophycins was straightforward: the structure is composed of four units (A-D, Figure 1); consequently several convergent approaches could be envisioned. The combination of two pairs of units (e.g., A-B and C-D) appeared to be optimally convergent. Since the success of the synthesis depended on the formation of a 16membered depsipeptide from an acyclic precursor, a macrolactamization involving the amino group of unit C and the carboxylate of unit B appeared to be the best choice. The acyclic precursor to cryptophycin D would therefore be 1. This, in turn, suggested a disconnection into two fragments, one represented by 2 and composed of (S)-( -)-2-hydroxy-4-methylvaleric (L-leucic) acid (D) and (R)-3-amino-2-methylpropanoic acid (C) units, and the other by 3 and composed of O-methylD-tyrosine (B) and (2E,7E,5S,6R)-5-hydroxy-6-methyl-8-phenyloctadienoic acid (A) units. In the direction of the synthesis, @ Abstract published in Advance ACS Absfracts, February 15, 1995. (1) Trimurtulu, G.; Ohtani, I.; Patterson, G. M. L.; Moore, R. E.; Corbett, T. H.; Valeriote, F. A.; Demchik, L. J. Am. Chem. SOC. 1994, 116, 47294731. (2) Schwartz, R. E.; Hirsch, C. F.; Sesin, D. F.; Flor, J. E.; Chartrain, M.; Fromtling, R. E.; Harris, G. H.; Salvatore, M. J.; Liesch, J. M.; Yudin, K. J. Ind. Microbiol. 1990, 5, 113-24. (3) Trimurtulu, G.; Ogino, J.; Heltzel, C. E.; Patterson, G. M. L.; Moore, R. E. Manuscript in preparation. An explanation of the structural misassignment for cryptophycins A and C is presented. (4) Heltzel, C. E.; Ogino, J.; Trimurtulu, G.; Mooberry, S. L.; Patterson, G. M. L.; Moore, R. E.; Corbett, T. H.; Valeriote, F. A,; Demchik, L. Manuscript in preparation. A Figure 1. Numbering system for each of the units of cryptophycins C and D. This numbering system is used for the NMR data.

The synthesis of unit A of the cryptophycins from (S)-trans-3-penten-2-ol and from (S)-trans-4-hexen-3-ol has been completed. The key stereodetermining step is a [2,3]-Wittig rearrangement of a propargyl ether. Elaboration of the rearrangement product was accomplished by means of a selective hydroboration-oxidation of a terminal alkyne, Horner-Emmons homologation of the derived aldehyde, followed by selective ozonolytic cleavage and Wittig olefination. This synthesis provides easy access to the series of cryptophycin analogues that incorporate a modified aromatic ring in unit A.

Four monocyclic precursors were assembled in the total synthesis of the proposed structure 1-A of (+)-tolyporphin A O,O-diacetate (X=Ac). Comparison of the spectroscopic data demonstrated that synthetic tolyporphin O,O-diacetate did not match the O,O-diacetate prepared from natural (+)-tolyporphin A (X=H), calling for a structural revision of this class of natural products. On the basis of a series of NMR experiments including synthetic intermediates, the structure of tolyporphin A is concluded to be 1-B, in which the configurations of quaternary centers C7 and C17 are opposite to those in the originally proposed structure.

Studies on the biosynthesis of cylindrospermopsin (1), a potent hepatotoxin associated with the cyanobacterium Cylindrospermopsis raciborskii, indicate that 1 is an acetogenin with guanidinoacetic acid serving as the starter unit of the polyketide chain. Feeding experiments show that C14 and C15 of 1 are derived from C1 and C2 of glycine, respectively, and C4 through C13 arise from five contiguous acetate units attached head to tail. The methyl carbon on C13 originates from the C1 pool. The starter unit, established by the incorporation of [guanidino-13C,α-15N]-guanidinoacetic acid into N16 and C17 of 1, does not appear to be formed from glycine by known amidination pathways. The origin of the NH−CO−NH segment in the uracil ring is also unknown.

Apratoxin A (1), a potent cytotoxin with a novel skeleton, has been isolated from the marine cyanobacterium Lyngbya majuscula Harvey ex Gomont. This cyclodepsipeptide of mixed peptide-polyketide biogenesis bears a thiazoline ring flanked by polyketide portions, one of which possesses an unusual methylation pattern. Its gross structure has been elucidated by spectral analysis, including various 2D NMR techniques. The absolute configurations of the amino acid-derived units were determined by chiral HPLC analysis of hydrolysis products. The relative stereochemistry of the new dihydroxylated fatty acid unit, 3,7-dihydroxy-2,5,8,8-tetramethylnonanoic acid, was elucidated by successful application of the J-based configuration analysis originally developed for acyclic organic compounds using carbon-proton spin-coupling constants ((2,3)J(C,H)) and proton-proton spin-coupling constants ((3)J(H,H)); its absolute stereochemistry was established by Mosher analysis. The conformation of 1 in solution was mimicked by molecular modeling, employing a combination of distance geometry and restrained molecular dynamics. Apratoxin A (1) possesses IC(50) values for in vitro cytotoxicity against human tumor cell lines ranging from 0.36 to 0.52 nM; however, it was only marginally active in vivo against a colon tumor and ineffective against a mammary tumor.

The isolation and total structure determinations of nostopeptolides A1 (1), A2 (2) and A3 (3) are described. These cyclic depsipeptides, which are devoid of cytotoxic, antifungal and inhibition of protease activities, are characteristic constituents of the cryptophycin-producing cyanobacterium Nostoc sp. GSV224. Structure elucidation by NMR analysis was made more challenging by the existence of each nostopeptolide in three conformations. One-dimensional TOCSY experiments proved to be very useful in isolating and identifying the nine amino acid residues and the butyryl group in each compound. The absolute stereochemistries of 1–3 were determined by comparing the amino acids in the acid hydrolyzates directly with authentic standards.

A total synthesis of cryptophycin 337 (1-aza-cryptophycin 1, 2 ), an analogue of the potent antitumor antibiotic cryptophycin 1 ( 1 ), is described. Cryptophycin 337 was unstable and underwent an unexpected skeletal rearrangement. ( R )-Mandelic acid was used as the sole source of asymmetry for the synthesis of unit A.