Generation and reactions of butadienylthionium ions from 2-vinylcyclopropyl sulfoxides under Pummerer conditions

摘要

J. CHEM. SOC. PERKIN TRANS. 1 1995 Generation and reactions of butadienylthionium ions from 2-vinylcyclopropyl sulfoxides under Pummerer conditions Tadashi Kataoka,* Harutoshi Matsumoto, Tetsuo Iwama, Taizo Ito and Hiroshi Shimizu Gijiu Pharmaceutical University, 6-1, Mitahora-higashi 5-chome, Gifu 502, Japan Treatment of 2-vinylcyclopropyl sulfoxides lacking an a-hydrogen with acid anhydrides produced butadienylthionium ion intermediates to give cyclic or acyclic conjugated dienes. The Pummerer reaction of sulfoxides eqn. (l) is a useful method for the synthesis of a-substituted or a,p-unsaturated sulfides and has been significantly studied from both R1-s-m2R2 + R3-C-O-C-R3 I1 I1+ II 000 R1S-CH-O-C-R3 + R3C02H II A2 0 mechanistic and synthetic points of view.' The general mechanism of the reaction is believed to consist of four sequential steps involving a thionium ion intermediate. Generation of vinylthionium ions has been widely investigated in the Pummerer reactions of ally1 sulfoxides3 and in the vinylogous Pummerer reactions of vinyl sulfoxides.3c*e*4 In contrast, little attention has been paid to generation of butadienylthionium ions by the Pummerer reaction of sulfoxides. The butadienylthionium ions are very versatile for the synthesis of functionalised conjugated dienes and we intend to explore a new method to generate them. It is known that sulfoxides lacking an a-hydrogen and carrying p-ones undergo the abnormal Pummerer reaction via sulfenic acid derivatives formed by p-hydrogen abstraction and C-S bond cleavage.6 If 2-vinylcyclopropyl sulfoxides I without an a-hydrogen are treated with acid anhydrides, a butadienylthionium ion II would be generated by the destruction of a cyclopropane ring accompanied by P-hydrogen elimination (Scheme 1, route a) 0 A2JJyt Ac~O -0Ac R2 __c I1RqI rather than a vinylcyclopropene by the abnormal Pummerer reaction (route b) because of the high energy content of the three-membered ring.For this strategy, we selected as substrates, 1,4-benzothiazin-3-one 1 -oxides without an a-hydrogen, because they were easily prepared from 4-methyl- 1,4- benzothiazin-3-one in good yields by vinylcyclopropanation * followed by m-chloroperbenzoic acid (MCPBA) oxidation.In this communication, we report the novel generation of butadienylthionium ion intermediates by the reactions of 2- vinylcyclopropyl sulfoxides lacking an a-hydrogen with acid an- hydrides. Several 2-vinylcyclopropyl sulfoxides 1 were treated under Pummerer conditions: Method A, 2 equiv. of trifluoroacetic Me a R1=R2=Me b R'=R2=Ha;pR2cR1=H,R2=Me-i d R1=Me,R2=H R' e R1=R2=Ph 0 f R1 = R2 =p-CCamp; 1 2 3 + 4 X=H 5 X-Ac Scheme 2 Reagents and conditions: i, Method A: 2 equiv. of TFAA, CH,CI,, room temp., 2 h; Method B: 5 equiv. of Ac,O, p-Me-C,H,SO,H (cat.), benzene, 85 OC, sealed tube, 24 h anhydride (TFAA) in CH2C12 at room temperature for 2 h; Method B: 5 equiv. of Ac20 and catalytic amount of p-MeC,H,SO,H in benzene at 85deg;C in sealed tube for 24 h (Scheme 2).The results are summarised in Table 1. Reactions of disubstituted vinylcyclopropanes, la (R' = R2 = Me), le (R' = R2= Ph) and If (R'= R2= p-Cl-C6H,), with 2equiv. of TFAA (Method A) furnished the cyclic dienes, 2a and 3a, 2e and Zf,in moderate yields, respectively (entries 1, 7 and 8). On the other hand, treatment of un- or mono-substituted vinylcyclopropanes, lb (R' = R2 = H), lc (R' = H, R2 = Me) and ld (R'= Me, R2 = H), afforded acyclic conjugated dienes, 4b or 5b, 4c and 4d, respectively (entries 3-6). The dienol 4 was obtained by hydrolysis of corresponding dienyl trifluoroacetates initially formed during work-up. A mixture of lc and Id (ca. 2: 1)tvSb when allowed to react by Method A, provided 4c in 70 yield.However, 4,which was observed in the 'H NMR spectrum of crude products, could not be isolated becuase of its instability to silica gel (entry 5). The geometry at the C(2')--C(3') double bond (A2'g3') of the acyclic dienes 4 and 5 t A mixture of 2-vinylcyclopropyl sulfoxides lc and Id (ca. 2: 1) was prepared as follows: 4-methyl-l,4-benzothiazin-3-onewas chlorinated with N-chlorosuccinimide, and the resultant a-chloro sulfide was treated with AgClO, in the presence of isoprene and then triethylamine followed by MCPBA oxidation. Table 1 Reactions of 2-vinylcyclopropyl sulfoxides 1 with acid anhydrides Products Entry Sulfoxides Conditions a ( yields) la 2a (25), 3a (46) la 2a (21), 3a (19) lb 4b (E, 81)dlb 5b (E, 39)d lc:ld(ca.2:l)' 4c (E, 70),' 4d (-)' Id 4d(E:Z = 1:1, 62)g le 2e (54)If 2f (60) Method A: 2 equiv.of TFAA, CH,Cl,, room temp., 2 h; Method B: 5 equiv. of Ac,O, p-MeC,H,SO,H (cat.), benzene, 85 OC, sealed tube, 24 h. Isolated yields unless otherwise mentioned. 'The ratio was determined by 'H NMR. The geometry of A2'q3 was determined from the coupling constant in 'H NMR. No geometrical isomer was isolated. 'The geometry of was determined by NOE. No geometrical isomer was isolated. Compound 4d was observed in the H NMR spectrum of crude products, but decomposed during purification by preparative TLC on silica gel. Crude yield. The geometry of A2'v3'was determined by NOE and the ratio was estimated by 'H NMR.was determined from the coupling constant between 2'-H and 3'-H (4b: J 15, 5b: J 15) in the 'H NMR spectrum or by the NOE technique (4c,a),and the geometry of A','' of 4 and 5, which consist of a single isomer, cannot at present be confirmed. In no case was any abnormal Pummerer-type product isolated. A possible mechanism for this Pummerer reaction is illustrated in Scheme 3. A butadienylthionium ion B generated by ring-opening of a cyclopropane ring with the elimination of a ring proton from an oxysulfonium salt A, which is initially formed by the reaction of a sulfoxide 1 with an acid anhydride. In the case of R' and/or R2 = H, the intermediate B might follow either of two pathways to give an acyclic diene. One is nucleophilic attack process (a) of XO-at the amp;-carbon of B to provide a trifluoroacetate 4', which is hydrolysed to the corresponding dienol 4 during work-up, or an acetate 5.The other is an intramolecular 2' + 41 polar cycloaddition 'v9 process (b). The resultant bicyclic sulfonium salt C reacts with XO-to give an acyclic diene, whose double bond may be isomerised under reaction conditions. In other cases (R' = R2 = Me or Ar), a cyclic diene 2 is formed via a carbocation D generated by the olefinic cyclisation" of a thionium ion B. When both R' and R2are methyl groups, deprotonation of a methyl hydrogen from the carbocation D affords another cyclic diene 3. The diene 3 may be given via processes of ring-opening of an oxysulfonium salt A' accompanied by methyl-proton abstraction, olefinic cyclisation of a thionium ion E and deprotonation of a carbocation F.In summary, reactions of 2-vinylcyclopropyl sulfoxides lacking an a-hydrogen with acid anhydrides such as TFAA or Ac,O provided cyclic or acyclic conjugated dienes via butadienylthionium ions, Extensive studies on this hmmerer reaction of various 2-vinylcyclopropyl sulfoxides are in progress. Experimental General procedure for the Pummerer reaction MethodA. To a solution of the 2-vinylcyclopropyl sulfoxide 1 (0.5 mmol) in dry CH,CI, (5 cm3) was added TFAA (210 mg, 1.0 mmol) at room temperature. After 2 h, saturated aqueous NaHCO, was added to the reaction mixture. The organic layer was separated and the aqueous layer was extracted with CH2C12 (2 x 5 cm3).The combined organic layer and extracts were dried (MgSOJ and concentrated. The residue was purified J. CHEM. SOC. PERKIN TRANS. 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