The squalestatins: Tricyclic 3,4-β-lactone and 3,4-oxetane systems

Article abstract of DOI:10.1016/S0040-4039(00)01286-7

Squalestatin 3,4-β-lactone-4,5-dimethyl ester (8) was reductively ring-opened to yield squalestatin 3-hydroxymethyl-4,5-dimethyl ester (6) using mild reducing conditions (sodium borohydride). Similarly, squalestatin 3,4-oxetane-4,5-dimethyl ester (10) was

Full text of DOI:10.1016/S0040-4039(00)01286-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 7547–7550
The squalestatins: tricyclic 3,4-b-lactone and 3,4-oxetane
systems
Brian Cox,* Nicolas Morley, Panayiotis A. Procopiou, Peter J. Sharratt, Nigel S. Watson
and Deborah Wild
Departments of Medicinal Chemistry 1 & 2, Glaxo Wellcome Research and Development,
Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire SG1 2NY, UK
Received 19 July 2000; accepted 27 July 2000
Abstract
Squalestatin 3,4-b-lactone-4,5-dimethyl ester (8) was reductively ring-opened to yield squalestatin
3-hydroxymethyl-4,5-dimethyl ester (6) using mild reducing conditions (sodium borohydride). Similarly,
squalestatin 3,4-oxetane-4,5-dimethyl ester (10) was found to ring-open to 3-iodomethyl squalestatin (15)
under the conditions used to cleave the methyl ester functions (lithium iodide/2,4,6-trimethylpyridine).
© 2000 Elsevier Science Ltd. All rights reserved.
Squalestatin 1, a member of a family of novel fungal metabolites, is a potent inhibitor of
squalene synthase (SQS), with an IC50 value of 12 nM against rat enzyme.1 Workers at Merck
isolated zaragozic acid A, which is identical to squalestatin 1.1 Extensive studies have been
undertaken in order to identify the structural features necessary for SQS inhibitory activity and
a number of reports have detailed chemistry undertaken around the 3 and 4 positions of the
ring,1 including the synthesis and biological evaluation of the 3-hydroxymethyl derivative of 1
and a series of close analogues.2,3 We report herein some interesting findings around the
formation and reactivity of b-lactones and oxetanes with potential application to other systems.
The 3-hydroxymethyl compound 2 is routinely prepared in a four-step process from 1,2 by
esterification (to 3), selective methyl ester hydrolysis at the 3-position (to 4), activation as the
3-N-hydroxysuccinimidyl ester 5 and selective reduction of the 3-carboxylate function by in situ
treatment with sodium borohydride to give 6 (55% yield 5 to 6) (Scheme 1). Alternatively, the
3-N-hydroxysuccinimidyl ester 5 can be isolated, purified by column chromatography and then
treated with sodium borohydride. Selective cleavage of the methyl ester groups within 6 is
achieved under very mild conditions by treatment with lithium iodide in 2,4,6-trimethylpyridine
at 45°C under a stream of nitrogen, resulting in the desired dicarboxylic acid 2.4 An alternative
* Corresponding author. Tel: +44 (0) 1438 764186; fax: +44 (0) 1438 763616; e-mail: bc5958@glaxowellcome.co.uk
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01286-7

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method for reduction of the 3-carboxylic acid 4 involves its activation using 2-chloro-3-ethylben-
zoxazonium tetrafluoroborate, followed by addition of sodium borohydride in THF, to give 6
in ꢀ25% yield. Interestingly, during the formation of the presumed carboxylate species 7, prior
to the addition of the sodium borohydride, the generation of a less-polar product was observed
on TLC. Repeating the procedure with work-up after the first stage allowed the isolation and
structural elucidation of this product, which was found to be the 3,4-b-lactone 8.5,6 Treatment
of the b-lactone 8 with sodium borohydride resulted in the formation of the 3-hydroxymethyl
compound 6. This suggests that the b-lactone is formed as an intermediate in the one-pot
reduction using 2-chloro-3-ethylbenzoxazonium tetrafluoroborate. In the literature, there are
only two references7 to the use of sodium borohydride for the reduction of b-lactones to
1,3-diols, which is normally accomplished by the use of more powerful reducing agents such as
lithium aluminium hydride. Workers at Merck have also described8 the preparation of a
3,4-b-lactone employing benzene sulfonyl chloride as an activating agent.
Scheme 1. (a) MeI, NaHCO3, DMF, 90%; (b) 0.1 M aq. NaOH, THF, 100%; (c) (i) N-hydroxysuccinimide,
1-cyclohexyl-3-(2-morpholinoethyl) carbodiimide metho-p-toluenesulfonate, THF, (ii) NaBH4, THF, 55%; (d) LiI
(anhydrous), 2,4,6-trimethylpyridine, 45°C, 48%; (e) (i) 2-chloro-3-ethylbenzoxazonium tetrafluoroborate, Et3N,
DCM, (ii) NaBH4, THF, 25%
Studies directed towards the derivatisation of the 3-hydroxymethyl compound 6 revealed
further unexpected chemistry around the 3 and 4 positions. Firstly, when evaluating approaches

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to the 3-methyl compound, the 3-(4-methylbezenesulfonate) 99 was subjected to a variety of
reducing conditions, and surprisingly the only product from these reactions was the 3,4-oxetane
derivative 10 (Scheme 2). The ease of formation of this product was confirmed by treatment of
9 with base.6 Furthermore, during the preparation of the 3-thiomethyl ether 12, by the treatment
of the 3-trifluoromethylsulfonate 1110 with sodium thiomethoxide, formation of the oxetane 13
was observed as a significant side product (Scheme 3). Attempts to deprotect the oxetane 10 to
give the di-acid using our standard lithium iodide conditions described earlier gave another
surprising product, the 3-iodomethyl, di-acid 15 resulting from the ring opening of the oxetane
(Scheme 2).11 The conditions usually associated with ether cleavage, even cyclic ethers, are
relatively harsh, e.g. aq. HBr or aq. HI at elevated temperature. However, there have been
several examples reported of oxetane ring opening using iodides, e.g. silyl iodides12 (TMS iodide
generated in situ from TMS chloride and sodium iodide) or by Lewis acid mediated methods13
(tetraethylammonium iodide/BF3 etherate).
Scheme 2. (a) Zn/NaI, DME (11% yield of 10) or Bu3SnH/NaI (37% yield of 10); (b) t-BuOK, THF (30% yield of
10); (c) LiI (anhydrous), 2,4,6-trimethylpyridine, 45°C, 13%
Scheme 3. (a) NaSCH3, DMF
The relative ease of reduction of 8 and the ring-opening of 10, both under mild conditions,
may be attributed to the relief of strain and steric interaction associated with the 4,6,5 ring
system, further studies are planned to investigate whether these findings can be extended into
other systems.
Acknowledgements
We are indebted to Mr. A. Roberts and Dr. R. Fletton for conducting NMR experiments. We
would also like to thank the Nuffield foundation for the provision of a Nuffield Science Bursary
to N.M.

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