Reversible Charge-Accelerated Oxy-Cope Rearrangements

Abstract

An asymmetric synthesis of the oxetane-containing norbornanone 23 and its coupling to trans-1-propenyllithium to give 24 are reported, in tandem with the preparation of the related alcohols 28 and 30. All three divinyl carbinols undergo anionic oxy-Cope rearrangement very rapidly at low temperature. Quenching of 24 -K + and 28 -K + under these conditions with water or various aqueous salt solutions results in protonation of the alkoxides. If these reaction mixtures are poured instead onto cold (0°C) silica gel, their sigmatropically related ketones are isolated in very good yield. Whereas the 24 -K + a 25 -K + equilibrium pair is not reactive to molecular oxygen, 30 -K + is directly converted into an R-hydroperoxy ketone under comparable conditions. These and additional observations are rationalized in the context of atropisomerism involving conversion of oxygen-up enolates, formed reversibly under kinetically controlled conditions, into their thermodynamically favored, more reactive oxygen-down conformers.

Synthesis and Evaluation of 1,5-Dithia-d-laminaribiose, Triose, and Tetraose as Truncated β-(1→3)-Glucan Mimetics

“…Synthetic efforts began with the large scale preparation of pentaacetyl 5-thia-D-glucopyranose essentially following the method of Whistler. 68 Thus, 1,2;5,6-di-Oisopropylidene-D-glucofuranose was converted to 3-O-acetyl-1,2-O-isopropylidene-D-glucofuranose (11) by known methods 69 and then exposed to a 10% molar excess of benzoyl Finally, isopropylidene removal with aqueous acetic acid, followed by Zemplen deesterification, and peracetylation gave pentaacetyl 5-thia-D-glucopyranose 15 72 in 60% yield (Scheme 1).…”

Nucleophilic Acyl Substitutions of Anhydrides with Protic Nucleophiles Catalyzed by Amphoteric, Oxomolybdenum Species.

“…Data: 1 H NMR (400 MHz, CDCl Dicetone-D-glucose-3-acetate 57 and 6-acetate 58 In a dry 25-mL, two-necked, round-bottomed flask was placed MoO 2 Cl 2 (1.9 mg, 0.01 mmol) in 5 mL of anhydrous CH 2 Cl 2 . To the above solution, acetic anhydride (140 µL, 1.5 mmol) was added at ambient temperature and stirred for one hour.…”

The stereoselective synthesis of oxetanes; exploration of a new, Mitsunobu-style procedure for the cyclisation of 1,3-diols

“…The organic extract was dried ((MgSO 4 ) and the solvent removed under reduced pressure to yield oxetane 63 (85 mg, 79%), R f (light petroleum-ether 4 : 1) 0.45; ν max (CHCl 3 )/ cm Ϫ1 965 (CO) and 910 (CO); δ H (500 MHz; CDCl 3 ) 7.45 (2 H,d,J 7.3,7.40 (2 H,t,J 7.4,7.31 (1 H,t,J 7.2,5.26 (1 H,d,J 6.7,PhCHOH),4.75 (1 H, dd, J 8.0 and 5.9, CH A H B O), 4.43 (1 H, t, J 6.53, CH A H B O), 2.99 (1 H, sept, J 7.1, CHMe) and 1.34 (3 H,d,J 6.8,CHMe); δ C (101 MHz; CDCl 3 ) 142. 9,128.5,127.9,125.2,90.4,75.1,39.7 and 17.7;m/z(ϩEI)…”

“…The formation of the oxetane D-ring in Taxol proceeds in 85% yield, 1 Scheme 1 but with many substrates the thermodynamic and kinetic barriers that stand in the way of four-membered ring formation dominate and yields are moderate at best, particularly with acyclic diols. 9 The [2ϩ2]-photocycloaddition reaction of an alkene and an aldehyde (Patternó-Büchi reaction) has been used with considerable success. Bach and co-workers have reported the synthesis of oxetanes in good yields and high stereoselectivities, both relative and absolute.…”