Tungsten-catalyzed cycloisomerization of alkynyl alcohols including 8 provides only the endocyclic enol ether (11) as a key intermediate for the branched C-glycoside substructure (2) of altromycin B. A sequence of Stille cross-coupling reaction and regio- and stereoselective functional group transformations affords each C13-diastereomer of the branched C-arylglycoside (2a and 2b).

Altromycin B (1, Figure 1), a member of the family of pluramycin antibiotics isolated from a South African bushveld soil, was first reported to have selective antibiotic activities against Gram-positive bacteria in the early 1990s and later reported to possess anticancer activity including in vivo activity against P388 leukemia, as well as colon, lung, and ovarian tumors.¹ The structure of altromycin B has been elucidated primarily by NMR spectroscopy, and thus the absolute stereochemistry of each of the widely separated chiral subunits has not been unambiguously assigned. Along with the studies on the biological activities, interactions between altromycin B with DNA² and its metal complex³ have been reported. Despite the attractive biological activity of altromycin B and various congener natural products, none of the altromycin natural products have been prepared by total synthesis. Pasetto and Franck recently reported the synthesis of both possible C13 isomers (2a, 2b) of the northwest quadrant of altromycin B, beginning with D-glucose.⁴ However, their efforts to assign C13 stereochemistry on the basis of the NMR comparison of their synthetic compounds 2a and 2b with altromycin B (1) were unsuccessful because of the complexity of the natural product structure. In support of our ongoing studies directed at the total synthesis of altromycins, we report herein a different synthesis of substructures 2a and 2b arising from non-carbohydrate precursors.⁵

Figure 1 Retrosynthetic analysis.

Our retrosynthetic analysis of 2a and 2b (Figure 1) envisioned a cross-coupling reaction⁶ to form the C13 1,1-methylene linking the carbohydrate and aglycone sectors in 3, with the carbohydrate coupling product 4 arising from the product of tungsten-catalyzed alkynol cycloisomerization of 5.⁷

Our synthesis began with the known diol 6 (Scheme 1).^7a We observed regioselective formation of the methyl ether from the propargylic alcohol using cyclic stannylene activation of the diol, followed by silylation of the remaining alcohol and removal of the benzoate protective group with DIBAL reduction to afford alkynyl alcohol 7. Alternatively, protection of diol 6 with 2,2-dimethoxypropane in the presence of catalytic p-TsOH, followed by DIBAL reduction of the benzoate ester provided the cyclic acetonide-protected alkynol 8.

Scheme 1 Preparation of Alkynyl Alcohols 7 and 8

The tungsten-catalyzed cycloisomerization was initially conducted with substrate 7 (Table 1). In contrast to other alkynol substrates explored in our laboratory, the preparation of six-membered glycal 9 could be optimized to provide only 33% yield. The crude ¹H NMR spectrum of the product mixture suggested the formation of a minor amount of the exocyclic methylene regioisomer 10, but this byproduct could not be isolated.⁸ However, cycloisomerization of the acetonide-protected substrate 8 gave better results. After optimization of the tertiary amine base and choice of solvent,⁹ we obtained glycal 11 as the only regioisomeric product in good isolated yield.

Table 1 Cycloisomerizations of Alkynyl Alcohols 7 and 8

Glycal 11 with the acetonide protective group also proved ideal for conversion into the stannylated glycal 13 (Scheme 2), due to the tolerance of the acetonide upon reaction with tert-butyllithium as well as solubility for subsequent transformations, relative to silyl ether protected glycals including 9. Although the Stille cross coupling reaction between 13 and α-iodostyrene was initially irreproducible under the usual conditions recommended for stannyl glycal partners,¹⁰ especially on millimole scales, the synergistic effect of copper-(I) salts and fluoride ion¹¹ [Pd(PPh₃)₄ (2 mol %), CuI (10 mol %), CsF (2 equiv), DMF, 45 °C] provided a robust solution for this important transformation in our synthesis, affording C13-disubstituted diene 14 in 66% yield.¹² Dihydroxylation¹³ of 14 using AD-mix α or β regioselectively occurred at the less-substituted alkene and produced a separable mixture of diastereomers 15a and 15b (15a:15b = 0.9:1 with AD-mix α; 3.6:1 with AD-mix β).

Scheme 2 Synthesis of Diols 15a and 15b

Treatment of diol 15a with borane·THF followed by NaOH/H₂O₂ oxidation provided triol 16 in 75% yield (Scheme 3), with stereochemistry of 16 consistent with sterically controlled addition of borane to the convex face of the acetonide-protected glycal 15a. However, the other diastereomer 15b produced a hydroxyl group syn to the adjacent acetonide protected group as the major product under the same reaction conditions. We postulate that the primary C19-alcohol may be directing the stereochemistry of hydroboration,¹⁴ since the mono-TBS ether 17 provided the expected stereoisomer 18 consistent with sterically controlled addition.

Scheme 3 Synthesis of Diols 20a and 20b

Compounds 16 and 18 were separately treated with TBSCl and imidazole to afford bis-TBS-protected compounds 19a and 19b, for which the primary alcohols were then selectively deprotected with catalytic CSA to provide 20a and 20b.¹⁵

The synthesis of the altromycin branched glycoside substructure 2a was completed from diol 20a, beginning with Parikh–Doering oxidation followed by I₂/KOH oxidation of the intermediate aldehyde to the methyl ester 21a (Scheme 4).¹⁶ Sequential removal of acetonide and silyl ether protective groups resulted in only trace formation of the tetraolmethyl ester 22a but rather the bicyclic lactone 23a as the major product. The rigidity of the fused bicyclic structure differentiated the remaining secondary alcohols of 23a so that the C4′ equatorial alcohol was regioselectively converted into the methyl ether 24a via the cyclic stannylene intermediate.¹⁷ Opening of the lactone to the methyl ester of target compound 2a was initially accomplished by acidic methanolysis (Amberlyst-15, MeOH), but better yields were consistently obtained under basic conditions.¹⁸ An identical series of transformations was conducted from the diastereomeric diol 20b to provide 2b.

Scheme 4 Conversion of Diols 20a and 20b to the Corresponding Altromycin Branched Glycoside Substructures 2a and 2b

The C13 hydroxyl stereochemistry was confirmed by X-ray crystallography of intermediate 23b, and NOE studies with diastereomer 23a allowed unambiguous stereochemical assignments for all synthetic products.¹⁹ The ¹H NMR spectra for our synthetic compounds 2a and 2b matched the data published by Pasetto and Franck for these two compounds.⁴

In combination with concurrent research into the synthesis of the altromycin aglycone,⁵ further studies directed toward the total synthesis of the altromycin natural products are in progress.

We thank the National Institutes of Health (CA 59703) for support of this research. We also acknowledge use of shared instrumentation provided by grants from the National Institutes of Health, National Science Foundation, and the Georgia Research Alliance (NMR spectroscopy, mass spectrometry, X-ray diffractometry) as well as the University Research Committee of Emory University (polarimeter). We also appreciate the work of Dr. Kenneth I. Hardcastle and Mr. Xikui Fang for crystal structure determinations of 23b and 24b