Asymmetric Induction by a Remote Chiral Substituent – Computationally Determined Stereodifferentiation in Michael Additions of α-Lithiated Allyl Sulfones

The Synthesis and Base-Induced Breakdown of Triaryl 1,4-Oxathiins-An Experimental and DFT Study

1,4-Oxathiins are valued for a breadth of bioactivities and are known commercial fungicides. This article explores a novel preparation of 2,3,6-trisubstituted 1,4-oxathiin-S,S-dioxides via the reaction of benzyl 1-alkynyl sulfones and aryl aldehydes under basic conditions. A total of 20 examples possessing exclusively a trans arrangement of the 2,3-diaryl substituents are exhibited; the products demonstrate a variation of functional groups on the aryl ring attached to the heterocyclic ring system. The preparation is hindered by the base sensitivity of the products, and a ring-opened by-product typically contaminates the reaction mixture. A DFT assessment of the overall system includes a lithium counterion and offers possible pathways for the incorporation of the aldehyde, the cyclization step and the requisite proton transfers. In addition, the DFT work reveals options for the ring opening chemistry. It appears the trans 2,3-diaryl selectivity is set during the cyclization stage of the reaction sequence. The practical work uncovers a new reaction pathway to create a family of novel 1,4-oxathiin-S,S-dioxides whereas the computational work offers an understanding of the structures and possible mechanisms involved.

Chiral Lithiated Allylic α-Sulfonyl Carbanions: Experimental and Computational Study of Their Structure, Configurational Stability, and Enantioselective Synthesis

X-ray crystal structure analysis of the lithiated allylic α-sulfonyl carbanions [CH2 CHC(Me)SO2 Ph]Li⋅diglyme, [cC6 H8 SO2 tBu]Li⋅PMDETA and [cC7 H10 SO2 tBu]Li⋅PMDETA showed dimeric and monomeric CIPs, having nearly planar anionic C atoms, only OLi bonds, almost planar allylic units with strong CC bond length alternation and the s-trans conformation around C1C2. They adopt a C1S conformation, which is similar to the one generally found for alkyl and aryl substituted α-sulfonyl carbanions. Cryoscopy of [EtCHCHC(Et)SO2 tBu]Li in THF at 164 K revealed an equilibrium between monomers and dimers in a ratio of 83:17, which is similar to the one found by low temperature NMR spectroscopy. According to NMR spectroscopy the lone-pair orbital at C1 strongly interacts with the CC double bond. Low temperature (6) Li,(1) H NOE experiments of [EtCHCHC(Et)SO2 tBu]Li in THF point to an equilibrium between monomeric CIPs having only OLi bonds and CIPs having both OLi and C1Li bonds. Ab initio calculation of [MeCHCHC(Me)SO2 Me]Li⋅(Me2 O)2 gave three isomeric CIPs having the s-trans conformation and three isomeric CIPs having the s-cis conformation around the C1C2 bond. All s-trans isomers are more stable than the s-cis isomers. At all levels of theory the s-trans isomer having OLi and C1Li bonds is the most stable one followed by the isomer which has two OLi bonds. The allylic unit of the C,O,Li isomer shows strong bond length alternation and the C1 atom is in contrast to the O,Li isomer significantly pyramidalized. According to NBO analysis of the s-trans and s-cis isomers, the interaction of the lone pair at C1 with the π* orbital of the CC double bond is energetically much more favorable than that with the "empty" orbitals at the Li atom. The C1S and C1C2 conformations are determined by the stereoelectronic effects nC -σSR * interaction and allylic conjugation. (1) H DNMR spectroscopy of racemic [EtCHCHC(Et)SO2 tBu]Li, [iPrCHCHC(iPr)SO2 tBu]Li and [EtCHC(Me)C(Et)SO2 tBu]Li in [D8 ]THF gave estimated barriers of enantiomerization of ΔG(≠) =13.2 kcal mol(-1) (270 K), 14.2 kcal mol(-1) (291 K) and 14.2 kcal mol(-1) (295 K), respectively. Deprotonation of sulfone (R)-EtCHCHCH(Et)SO2 tBu (94 % ee) with nBuLi in THF at -105 °C occurred with a calculated enantioselectivity of 93 % ee and gave carbanion (M)-[EtCHCHC(Et)SO2 tBu]Li, the deuteration and alkylation of which with CF3 CO2 D and MeOCH2 I, respectively, proceeded with high enantioselectivities. Time-dependent deuteration of the enantioenriched carbanion (M)-[EtCHCHC(Et)SO2 tBu]Li in THF gave a racemization barrier of ΔG(≠) =12.5 kcal mol(-1) (168 K), which translates to a calculated half-time of racemization of t1/2 =12 min at -105 °C.

Optimization of peptidyl allyl sulfones as clan CA cysteine protease inhibitors

This research investigates the synthesis and inhibitory potency of a series of novel dipeptidyl allyl sulfones as clan CA cysteine protease inhibitors. The structure of the inhibitors consists of a R(1)-Phe-R(2)-AS-Ph scaffold (AS = allyl sulfone). R(1) was varied with benzyloxycarbonyl, morpholinocarbonyl, or N-methylpiperazinocarbonyl substituents. R(2) was varied with either Phe of Hfe residues. Synthesis involved preparation of vinyl sulfone analogues followed by isomerization to allyl sulfones using n-butyl lithium and t-butyl hydroperoxide. Sterics, temperature and base strength were all factors that affected the formation and stereochemistry of the allyl sulfone moiety. The inhibitors were assayed with three clan CA cysteine proteases (cruzain, cathepsin B and calpain I) as well as one serine protease (trypsin). The most potent inhibitor, (E)-Mu-Phe-Hfe-AS-Ph, displayed at least 10-fold selectivity for cruzain over clan CA cysteine proteases cathepsin B and calpain I with a (kobs)/[I] of 6080 ± 1390 M(-1)s(-1).

Asymmetric tandem Michael addition-ylide olefination reaction for the synthesis of optically active cyclohexa-1,3-diene derivatives

The reaction of a crotonate-derived chiral phosphonium salt with alpha,beta-unsaturated carbonyl compounds in the presence of Cs(2)CO(3) affords optically active cyclohexa-1,3-diene derivatives with up to 90% ee in good yields.

The role of an aromatic group in remote chiral induction during conjugate addition of alpha-sulfonylallylic carbanions to ethyl crotonate

The impact of a remote aromatic nucleus on the stereochemical outcome of the conjugate addition of α-sulfonylallylic carbanions to an α,β-unsaturated ester was investigated. α-Regioselectivity coupled with anti-diastereoselectivity is accompanied by a prominent preference for relative configuration 3 over 4. The 9-anthryl moiety has shown itself greatly superior over all other groups in this bias. A lithium ion–aromatic π interaction has been postulated as decisive for the remote transmission of chirality.