01833nas a2200253   4500008004100000022001400041245015100055210006900206260007700275300001400352520086900366653002001235653002501255653001301280653002401293100001901317700001801336700001801354700001601372700001701388700002401405700002301429856012701452       2014                        eng d  a1434-193X00aSynthesis of alpha,beta-unsaturated delta-lactones by vinyl acetate mediated asymmetric cross-aldol reaction of acetaldehyde: mechanistic insights0 aSynthesis of alphabetaunsaturated deltalactones by vinyl acetate  aBOSCHSTRASSE 12, D-69469 WEINHEIM, GERMANYbWILEY-V C H VERLAG GMBHcAUG  a5247-52553 a<p>A tandem asymmetric cross-aldol reaction involving the in situ generation of acetaldehyde from vinyl acetate has been developed that may resolve the challenges associated with the handling of acetaldehyde. The simple protocol, mild reaction conditions and unique harmony of an organocatalyst with a biocatalyst make this method a valuable tool for the synthesis of asymmetric beta-hydroxy aldehydes. By using this methodology we have accessed alpha,beta-unstaurated delta-lactones as well as isochromenones with high enantioselectivities from both asymmetric beta-hydroxy aldehydes and ketones. Systemic density functional theory (DFT) studies were also performed to gain mechanistic insights into the role of hydrogen bonding in the asymmetric cross-aldol reaction of acetaldehyde and in the key cis/trans isomerisation step in the synthesis of d-lactones.</p>10aAldol reactions10aasymmetric catalysis10alactones10areaction mechanisms1 aKumar, Manjeet1 aKumar, Arvind1 aRizvi, Masood1 aMane, Manoj1 aVanka, Kumar1 aTaneja, Subhash, C.1 aShah, Bhahwal, Ali  uhttp://library.ncl.res.in/content/synthesis-alphabeta-unsaturated-delta-lactones-vinyl-acetate-mediated-asymmetric-cross-002012nas a2200217   4500008004100000022001400041245011600055210006900171260000800240300001500248490000700263520120100270653003601471653002301507653002901530653002001559653002401579100002501603700001701628856014901645       2023                        eng d  a1861-472800a"Weak" C-H•••S interaction drives enantioselectivity in cinchona alkaloid complex catalyzed thiocyanation0 aWeak CH S interaction drives enantioselectivity in cinchona alka  cJUL  ae2023003210 v183 a<p>
	The great success of asymmetric organocatalysis has made it one of the most important advancements made in chemistry in the past two decades. A significant achievement in this context is the asymmetric organocatalysis of the thiocyanation reaction. In the current study, computational studies with density functional theory have been done in order to understand an interesting experimental finding: the reversal of enantioselectivity from R to S when the electrophile is changed from beta-keto ester to oxindole for the thiocyanation reaction with the cinchona alkaloid complex catalyst. The calculations reveal an unusual fact - the principal reason for the reversal is the presence of the C-H center dot center dot center dot S noncovalent interaction, which is present only in the major transition states in each of the two nucleophile cases. Only recently has it been realized that the supposedly weak C-H center dot center dot center dot S noncovalent interaction has the properties of a hydrogen bond, and the fact that this interaction is the cause of enantioselectivity has relevance, because of the large number of asymmetric transformations involving the sulphur heteroatom.</p>
10aDensity functional calculations10aEnantioselectivity10anoncovalent interactions10aorganocatalysis10areaction mechanisms1 aBanerjee, Subhrashis1 aVanka, Kumar  uhttp://library.ncl.res.in/content/weak-c-h%E2%80%A2%E2%80%A2%E2%80%A2s-interaction-drives-enantioselectivity-cinchona-alkaloid-complex-catalyzed