Tetrahdmm Ltaas, Vol. 35 No. 6. pp. ~333-834.1994 Rlseviicrsciarelad

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0040-4039(93NIO352-K

Novel Method For The Preparation Of Momfluoroacetyl And Difluoro8cetytiran Derivatives

VirendraKuma+.PatrickMcCloskey,andMakolmRBell

Sterling Winthrop Phamuu~uticals Research Division, Collegevilk, Pennsylvania 19426 (USA)

Abstract: Monofluoroacetyl and difluoroacctylsteroidal furans were prepared by selective reductive

dehalogenation of chlorofluom and chlomdifluomacetyl inmm&iates with sodium formaldehyde sulfoxylate.

WC have mcently qcatedl the synthesis of S-mthylsulfonyl[3,2-b]~dat fumn (1) and the androgen

receptor activity of this compound. In order to explore the suuctum-activity relationship of this series of

compounds, we have introduced a number of other electron withdrawing substituents at 5’ position of the

furanosteroids.2 Among these, our interest was to prepare S- fluorinated ketones. The trifluoromtthyl ketones

could be readily synthesixed using the literamre pr~xdms.394 However thae arc not many shaight forward

methods repmted in the li~turc3~45 to prepare either monofl uomnethylketones or difl uofomcthylketones (2

and 3). Fluorinated ketones are the subject of renewed interest not only for use as inteunedlates to prepate other

fluculnated compounds but also as inhibitors of a variety of esterases and pmmases.3 We describe here a novel

two step procedure to prepare the title ketones from 17-unsubstituted fumnosteroids and sodium formaldehyde

sulfoxylate, an inexpensive selective reducing agent. Although this methodology was only applied to the

pmpamtion of S’-substituted monofluoro (It) and difluommethyl(12 and 13) stcroidal ketones, it may have a

mom general application.

l.R=CHI,R’=Sl&CH~ 2,R=H,R’=COCH# 3. R = H, CHa. R’ = CQCHF2

The requisite starting materials, steroidal[3,2-blfurans, were prepared by the procedure nqorted earlier.1

The mono-fluoroacetyl derivative was obtained following the reaction pathways as depicted below. Early

attempts to react the Y-lithio derivative (n-BuLiiMEDA/ether, CPC+2OoC) of I with methyl

chlorofluoroacetate to give chlorofluomacetyl derivative 8 resulted only in a minor amount of the desired

compound along with other products. Alternative bases such as set-BuLi, LDA, LiHMDS or KHMDS

similarly did not improve the yield of 6 and in all cases a mixture of products was obtained. The *H-NMR of

thecrude lnixturertvealedthatthcesmaswdlasthcchlorofraup~withrheanioa.

To circumvent these problems the amide 7 [bp 12513PC (ISmm), yield 80961 was prepared following

Weinreb’s procedure6 from methyl chlomfluoroacetate. The S-lithio derivative of J reacted smoothly with

amide 7 to give 6 (yield 84%). The tH-NMR of the crude material indicated only minor amounts of side

products and the desimd product 8 was easily purified by recrystallixation. Reductive dehalogenation of 8 with

833

834

sodium formaldehyde sulfoxylate (Rongalit@),utility of this reagent for dehalogenation and the Mchanism is

first described by Han@, in boiling ethanol or THF/ethanol (1: 1) gave the monofluca&etone lls (R’ = H)

aftez the hydrolysis of the TI-P in 80% yield Besides being inexpensive, the selectivity of the reagent was

rem&able since most other reducing agents &AH, DIBAL-H, lO%IWC) led to the desired vnd but

also gave considerable amounts (>25%) ofover Educed products.

5,R=W,R’=Ac 6,R=CH,.R’=Ac

ll.R=H.R’=H,X=H U,R=H.R’eAc,X=P 13.R=CHsR’=~.X=P

Difluoroketones were prep@ following a similar reaction pathway. However, we opted to utilize the

commuzUy available chlorodifluoroacetic anhydride. If desired. the amide similar to 7 could he easily

pxqared firan commcmUy available methyl cwuoraacaate and the FemainiDg mtcticn stqutncc follows

as above. Thus, to a solution of 17-0-acetylated derivatives 5 and c at OOC was added chlomdifluomacetic

anhydride dropwise. After stining for 16 h at room ttmpaanat followed by basic workup (10% aq NsHCO3)

9 and 10 were obtained in 84% and 80% yields, respectively. Reductive dehalogenation with sodium

forma+hyde sulfoxylate gave the difluoroketones 12 (yield 83%)9 and 13 (yield 86%).10

In summary we have developed a novel two step procedure to prepare monofluoroacetyl and

difluoroacetyl fumnos&midal derivatives which uses an inexpensive and selective reducing agent and avoids

the useof toxic fluorine containing reagents. This methodology should he readily adapted to prepare mono- or

difluorlnated ketones from other heterocyclic or non-heterocyclic compounds. Applications of this

methodology to prepare such ketones BTC in progress.

Acknowledgement: We thank the Department of Molecular Characterization for MS and NMR spectra.

References and Notes

1.

2. 3. 4.

: 7: 8.

9.

10.

Kumar, V. ; Dam. S. J. ; Rell, M. R. ; Alexan&, M. D. ; Chridansen. R. C. ; Ackman. J. H. ; K&&i. M. E.: Pilling, G. M. ; Hetrmann, Jr., J. L. ; Winneker, R. C. ; Wagner, M. M. Temhedron 1991.47.5099. Kumnr. V. Unpublished wurk. Roan, S. ; FI, R. Tetmhe&on 1985. II, 1111 and the references tkmin. Welch. J. T. Tetrahedron 1987.43.3123 ti tk &aeuces Mm. Bkgu’~, J.-. P. ; Bmnet-Ddpon. D. Tetrahdon 1991,47.3207 and the rcfacnccs thmin. N&m, S. ; Weinreb, S. M. Tamh&on Letters 1981,22.3815. Harris, A R. Synth. Conutwn. 1987.17, 1587. 11. mp 133-135% MS (CI) e 361. ‘H-NMR (CDCl3,300 MHz) 8 0.74 (s,3H), 0.89.21#) (m. 21H). 3.67 (t, J I 10 Hz, lH), 5.22 (d. JH~ = 47 Hz, 2H), 7.16 (s , 1H). 12. mp 183-185% MS (Cl) MH+ 423.lI-hNMR (CDCl3,300 MHz) 6 0.79 (s, 3H), 1X-2.75 (m. 22H), 2.01 (s. 3H). 4.56 (t, J = 9 Hz. II-I). 6.12 (t, JHP = 54 Hz). 7.27 (s. 1H). 13, mp 189-191%. MS (CI) MH+ 435. IH-NMR (CDCi3,300 MHz) 6 0.73 (s, 3H), 0.78 (s, 3H). 0.81-2.67 (m, 22H). 2.01 (s, 3H), 4.57 (t, J = 8 Hz, lH), 6.12 (1. Jm = 54 Hz. 1H). 7.28 (s, 1X-l).

(Received in USA 1 N~vtv&?r 1993; revised 18 November 1993; accepted 19 Novunbcr 1993)