Asymmetric Total Synthesis of Attenol A and B

Abstract

The asymmetric total synthesis of attenol A (1) and B (2), which possess challenging structures and an interesting biological activity, was accomplished in a convergent and highly stereo­selective manner (de, ee ≥ 96%) with good overall yield. The short total synthesis is based on asymmetric alkylations of SAMP-hydrazones as well as a Sharpless asymmetric dihydroxylation as key steps.

asymmetric dihydroxylation - dithianes - natural products - SAMP-hydrazone methodology - total synthesis

References

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4 The introduction of keto groups using dithianes as acylanion equivalents followed by intramolecular ketalization is a very common approach in spiroketal synthesis. For a review about spiroketals, see: Perron F. Albizati KF. Chem. Rev.  1989,  89:  1617 

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5a Enders D. In Asymmetric Synthesis   Vol. 3:  Morrison JD. Academic Press; Orlando: 1984.  p.275 

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Compound 6 is a versatile chiral dihydroxy acetone dicarbanion equivalent. For its applications see:

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9 Since the hydrazone cleavage of 8 proceeded without epimerisation, the ee of the anti-2,2-dimethyl-1,3-dioxan-5-one 4 is assumed to be at least as high as its de. The absence of epimerization can be proved on the stage of the anti-2,2-dimethyl-1,3-dioxane 10, whose ¹³C NMR resonance is at δ = 100.2 in accordance to Rychnovsky’s criteria. See: Rychnovsky SD. Rogers B. Yang G. J. Org. Chem.  1993,  58:  3511 

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12

Experiments towards higher stereoselectivities in the reduction step were not conducted since the newly formed stereogenic center had to be removed afterwards.

13

A large excess of Bu₃SnH was necessary to sufficiently reduce the occuring side reactions. Under optimized conditions 10 contained only 3 mol% of an isomerization product in which the terminal double bond had migrated between C-19 and C-20 (the numbering refers to the final natural products).

14 Ishikawa T. Ikeda S. Ibe M. Saito S. Tetrahedron  1998,  54:  5869 

15

The de of 13 and the ee of 5 were verified by HPLC on chiral stationary phase. In order to do so, the 1:1-epimeric mixture of 13 and the racemate of 5 had to be synthesized which was performed starting from the N,N-dimethyl-hydrazone of 12. Alkylation with MeI and ozonolysis gave the α-alkylated racemic aldehyde which was treated with SAMP and Ph₃PCHCO₂Et to obtain the desired mixtures of compounds.

16 Kolb HC. VanNieuwenhze MS. Sharpless KB. Chem. Rev.  1994,  94:  2483 

17

The ee of 15 was verified by HPLC on chiral stationary phase. For this, ent-15 had to be synthesized analogously to 15 starting from the RAMP-hydrazone ent-7 and performing the Sharpless asymmetric dihydroxylation of ent-5 with the AD-mix α.

18

The reaction sequence leading to 15 was also conducted starting with 5 of much lower enantiomeric purity (i.e. ee = 83%). After HPLC, 15 (obtained in lower yield) was still diastereomerically and enantiomerically pure (de, ee ≥ 98%) which indicates the high stereoselectivity of the Sharpless asymmetric dihydroxylation.

19 Posner GH. Weitzberg M. Hamill TG. Asirvatham E. He CH. Clardy J. Tetrahedron  1986,  42:  2919 

20

Our synthetic material was identical in all respects with physical and spectroscopic data provided for the natural products. Compound 1: [α]_D ²⁶ -8.2 (c 0.35, CHCl₃) {(ref. [2a] , [α]_D ²⁸ -9.7 (c 0.35, CHCl₃) and [α]_D ²⁸ -8.0 (c 0.38, CHCl₃) for natural 1. Compound 2: [α]_D ²⁶ 37 (c 0.072, CHCl₃) {(ref. [2a] , [α]_D ²⁹ 34 (c 0.073, CHCl₃) and [α]_D ²⁸ 31
(c 0.065, CHCl₃) for natural 2}.
All new compounds gave satisfactory spectral data and correct elemental analyses.