¹³C magnetic resonance studies. 120. The Simmons–Smith reaction with some silyl enol ethers. Unusual ring expansions of some norcamphorsti

Abstract

Unexpected difficulties with Simmons-Smith cyclopropanation of the tr~methylsilyl en01 ether of exo-tricyclo[3.2.1 .02.4]-octan-6-one led to a reexamination of this reaction with the parent norcamphor derivative which under "concentrated" conditions gave ring-expanded allylic ethers in low yield. With the tert-butyldimethylsilyl ethers, however, the allylic product is formed in high yield in the norcamphor and exo-trimethylenenorcamphor (tricyclo[5.2. l .0'.6]decan-8-one) systems. The tert-butyldimethylsilyl en01 ethers of bicyclo[3.2. Iloctan-6-one and nopinone do not undergo ring expansion under similar conditions but the silyl ether of 3-methylenenopinol is a major product in the latter case. ARTHUR J. RAGAUSKAS et J. B. STOTHERS. Can. J. Chem. 63, 2969.Les difficultCs inattendues qui se sont manifestees lors de la rCaction de cyclopropanation selon Simmons-Smith de 1'Cther Cnolique trimCthylsilyle de la tricyclo[3.2.1.02~4]octanone-6 exo nous ont incite a reetudier cette reaction avec le derive norcamphre apparent6 qui, lorsqu'il est soumis a des conditions "concentrCes", conduit avec un faible rendement aux Cthers allyliques ayant subi une expansion du cycle. Toutefois, avec les ethers tert-butyldimCthylsilyle du norcamphre et du trimCthyltnenorcamphre-exo (tricyclo[5.2.1 .02.6]dCcanone-8), on obtient le produit allylique avec un rendement elevt. Dans des conditions analogues, les ethers Cnoliques tert-butyldimethylsilyles de la bicyclo[3.2. Iloctanone-6 et de la nopinone ne subissent pas d'extension de cycle; toutefois, dans ce dernier cas, le produit principal est 1'Cther silylk du methylbne-3 nopinol.[Traduit par le journal] Introduction Some years ago we described an efficient synthetic method for ring expansion of several bicyclic ketones (1). Our sequence involves generation of the trimethylsilyl en01 ether followed by Simmons-Smith cyclopropanation and basecatalyzed cleavage of the cyclopropyl ether (Scheme 1). The cyclopropoxide produced by ether cleavage immediately undergoes homoketonization, which strongly favors ring expansion in [2.2.1], [2.2.2], and related systems. Subsequently this sequence was employed for ring expansion of a variety of polycyclic ketones (2-4). From these and other cases ( 3 , we have collected more than 25 examples of the Simmons-Smith reaction with polycyclic trimethylsilyl en01 ethers and, in each case, a single cyclopropanation product was obtained in good to high yield (70-95%) under a standard set of conditionsIn the course of a recent study (6), however, we found that, with our standard conditions, 6-trimethylsilyloxy-exo-tricyclo-[3.2.1.02.4]oct-6-ene (1) presented unexpected difficulties, such that with reflux times of 20 h or more, which are typical for good results with all other systems investigated to date (1-5), a mixture of products resulted that was too complex to warrant detailed examination. With a reflux time of 15 h, the reaction led to a product containing, by gas-liquid chromatography (glc), one major component (65%) and a minor component (-15%) toget...

Synthetic studies toward the kempane diterpenes. Diels–Alder additions to bicyclic dienes

“…Further experimentation with indirect approaches to methylation was carried out with a simpler tricyclic model compound 44 (Scheme 6). Although there are not many examples of cyclopropanation of TBDMS-enol ethers in the literature, 13 these Scheme 5 Attempted opening and cyclization of hemiacetal. Reagents and conditions: i, MeLi, Et 2 O; ii, 12, toluene, reflux, 2 days; iii, 5% aqueous HCl-THF, RT, 24 h. silyloxycyclopropanes can be converted to α-methyl ketones with base 14 or acid.…”

“…Reagents and conditions: i, MeLi, Et 2 O; ii, 12, toluene, reflux, 2 days; iii, 5% aqueous HCl-THF, RT, 24 h. silyloxycyclopropanes can be converted to α-methyl ketones with base 14 or acid. 13 Treatment of 44 and a large excess of CH 2 I 2 with Et 2 Zn gave the cyclopropanated compound 45 that had the desired stereochemistry at C-3a (from NOE experiments). Furthermore, monoreduction of 45 with a bulky hydride smoothly gave 46 with high chemo-and stereoselectivity.…”

“…For 47a: colourless viscous oil; ν max (Nujol)/cm Ϫ1 3421 (broad) and 1713; δ H 6.34 (1 H, d, J 1.2, 7-H), 4.93 (1 H, m, 6-H), 2.53-2.43 (m), 2.33-2.19 (m), 1.91-1.78 (m), 1.74 (3 H, s, 8-methyl), 1.43 (1 H, m), 1.28 (3 H, s, 9a-methyl), 0.90 (9 H, s, SiC(CH 3 ) 3 ), 0.04 (3 H, s, SiCH 3 ) and 0.02 (3 H, s, SiCH 3 ); NOE data: 4.93 (6.34, 4%; 1.28, 7%); δ C 202.7 (C-9), 144.1 (C-7), 138.1 (0), 134.2 (0), 121.4 (0), 67.9 (C-6), 50.9 (1), 50.2 (1), 47.9 (C-9a), 28.9 (2), 28.0 (2), 25.7 (SiC(C H 3 ) 3 ), 25.0 (2), 22.0 (9b-methyl), 18.0 (SiC(CH 3 ) 3 ), 15.9 (2-methyl), Ϫ4.0 (SiCH 3 ) and Ϫ4.1 (SiCH 3 ); m/z 317 (M ϩ Ϫ 45, 2%), 262 (4), 244 (8), 229 (6), 175 (11) (3 H, d, J 0.9, 8-methyl), 1.66 (1 H, dd, J 9.2 and 3.5), 1.55-1.41 (m), 1.34 (3 H, s, 9a-methyl), 1.24 (1 H, m), 0.93 (9 H, s, SiC(CH 3 ) 3 ), 0.12 (3 H, s, SiCH 3 ) and 0.11 (3 H, s, SiCH 3 ); δ C 202.7 (C-9), 144.7 (C-7), 137.7 (0), 135.2 (0), 121.1 (0), 66.7 (C-6), 49.5 (1), 49.0 (1), 45.3 (C-9a), 30.2 (2), 29.9 (2), 25.7 (SiC(C H 3 ) 3 ), 25.2 (9a-methyl), 25.1 (2), 22.5 (2), 18.1 (SiC(CH 3 ) 3 ), 16.1 (8-methyl), Ϫ4.0 (SiCH 3 ) and Ϫ4.2 (SiCH 3 ); m/z 362 (M ϩ , 1%), 317 (2), 264 (16), 149 (10), 138 (14), 98 (13), 75 (100) and 73 (23). To a solution of 47a (101 mg, 0.279 mmol) in dry dichloromethane (3.0 ml) at Ϫ78 ЊC under argon was added over 5 min a solution of 1,3-dithienium tetrafluoroborate (172 mg, 0.836 mmol) in dry nitromethane (1.0 ml).…”

“…Chromatography of the residue gave 52 (83. 5 (18), 149 (13), 135 (15), 119 (100), 112 (29) and 69 (15).…”

“…The solution was dried (Na 2 SO 4 ) and concentrated under reduced pressure. Preparative TLC provided 53 (18 (10), 205 (18), 178 (17), 165 (36), 151 (100), 137 (14), 123 (12), 91 (13) and 79 (15).…”