Syn! anti

Here we will illustrate the method using a single example. The aldol reaction between an enol boronate and an aldehyde can lead to four possible stereoisomers (Figure 11.32). Many of these reactions proceed with a high degree of diastereoselectivity (i.e. syn anti) and/or enantioselectivity (syn-l syn-Tl and anti-l anti-lT). Bernardi, Capelli, Gennari,... 

Tine force field was then used to predict the results for fhe addition of the E and Z isomers c Ihe enol boronate of butanone (R = Me) to ethanol (R = Me). The relevant transitio. Iructures are shown in Figure 11.34. A Boltzmann distribution, calculated at the ten perature of the reaction (—78°C), predicted that the Z isomer would show almost complel syn selectivity syn anti = 99 1) and that the E isomer would be selective for the an product anti syn = 86 14). These results were in good agreement with the experunenti... 

The syn-anti conformational problem of a- and /3-pyrazofurins (756 one of the rare naturally occurring pyrazole compounds, see Section 4.04.4.4.3), which involves a rotation around a pyrazolic sp carbon atom and a sugar sp carbon atom, has been studied theoretically using the PCILO method (81MI40403). In agreement with the experimental observations, the /3 anomer is energetically more favourable than the a anomer, the preferred conformations being anti and syn, respectively. 

Syn 3-(EthyltHloniethyl)-4-hydroxy-6-pt)enyl-2-hexanone (4) and anti (5) To ethanethiol (10 0 mg 0 17 mmol) in THF (2 mL) was added 1 54 M n butyltittnum m hexane (0 11 mL) at 0 C under Ar Stannous triflate (69 0 mg 017 nvnol) was added and alter 20 mm the mocture was cooled to 4S°C Methyl vinyl Ketone 1 (118 mg 1 98 mmol) m THF (1 5 mL) was added followed by 3 phenyiptopanal 3 350 mg 2 61 mmol) m THF (1 S mL) Alter 12 h aq cilnc acid was added and the organic material extracted with OHjClj The resxlue after evaporation was dissolved in MeOH and treated with citric acxt After 30 min stimng, the mixture was quenched with pH 7 phosphate buffer extracted with CH2CI2, the solvent evaporated and the residue chromatographed to afford 336 mg ot 4 (75%), syn anti (90 10)... 

A prototype of such phenomena can be seen in even the simplest carboxylic acid, acetic acid (CH3CHOOH). Acidity is determined by the energy or free energy difference between the dissociated and nondissociated forms, whose energetics usually depend significantly on their conformation, e.g., the syn/anti conformational change of the carboxyl-ate group in the compound substantially affects the acid-base equilibrium. The coupled conformation and solvent effects on acidity is treated in Ref. 20. 

Protonation of the anion [SN2] by acetic acid in diethyl ether produces the thermally unstable sulfur diimide S(NH)2. Like all sulfur diimides, the parent compound S(NH)2 can exist as three isomers (Scheme 5.5). Ab initio molecular orbital calculations indicate that the (cis,cis) configuration is somewhat more stable than the (cis,trans) isomer, while the (trans,trans) isomer is expected to possess considerably higher energy. The alternative syn,anti or E,Z nomenclatures may also be used to describe these isomers. The structures of organic derivatives S(NR)2 (R = alkyl, aryl) are discussed in Section 10.4.2. 

Bcnzil forms three dioximes which are distinguished by the names syn, anti, and amphi. ... 

An important aspect is the control of the stereochemical outcome.During the course of the reaction two new chiral centers can be created and thus two diastereomeric pairs of enantiomers (syn/anti resp. erythro/threo pairs) may be obtained. 

Scheme 9. The threo 2,3-diol problem. The erythro/threo notation is based on Fischer projection formulas. For example, if threo 2,3-diol 28, shown here in a staggered zigzag conformation, was depicted in an eclipsed Fischer projection, the adjacent hydroxyls attached to carbons 2 and 3 would reside on opposite sides of the carbon chain. An alternative, perhaps less ambiguous, descriptor is the syn/anti notation.63...

At 03C, a solution of 120 mg (1.2 mmol) of phenylacetylene in 2 mL of dry Et20 is treated with 0.45 mL (1.2 mmol) of 2.6 M BuLi in hexane, The mixture is stirred for 30 min, then a solution of 290 mg (1.3 mmol) of anhyd zinc bromide in 2 mL of Et20 is added. After cooling to —78 C, 100 mg (0.6 mmol) of 2-(ben-zyloxy)propanal (7) are added and the reaction mixture is allowed to warm to 0 C over a 2-h period. The reaction is quenched with sat. aq NH4C1. The organic layer is separated, dried over Na2S04 and concentrated. The residual oil is purified by column chromatography on silica gel yield 154 mg (95%) d.r. (syn/anti) 95 5 (determined by HPLC). 

---