Stereoisomer analogs

Methyl-l-phenyl-2(17/)-quinoxalinone (226) and methyl methacrylate (227) gave the photoadduct, methyl l,2a-dimethyl-3-oxo-4-phenyl-2,2a,3,4-tetra-hydro-l//-azeto[l,2-fl]quinoxaline-l-carboxylate (228) as a separable mixture of two stereoisomers (CH2CI2, MeOH, hv, N2, <15 h 40% each) analogs likewise. " ... 

Biocatalytic access to both antipodal sulfoxides was exploited in total syntheses of bioactive compounds, which is outlined in some representative examples. Biooxidation of functionalized dialkyl sulfides was utilized in the direct synthesis of both enantiomers of sulforaphane and some analogs in low to good yields and stereoselectivities (Scheme 9.27) [206]. This natural product originates from broccoli and represents a potent inducer of detoxification enzymes in mammalian metabolism it might be related to anticarcinogenic properties of plants from the cruciform family. All four possible stereoisomers of methionine (R = Me) and ethionine sulfoxides... 

The analogy goes like this when yon have a lot of stereocenters in a com-ponnd, there will be many stereoisomers (brothers and sisters). Bnt, they will be paired up into sets of enantiomers (twins). Any one molecule will have many, many diastereomers (brothers and sisters), bnt it will have only one enantiomer (its mirror image twin). For example, consider the following compound ... 

With some 1,3-dipoles, two possible stereoisomers can be formed by syn addition. These result from two differing orientations of the reacting molecules that are analogous to the endo and exo TS in D-A reactions. Phenyldiazomethane, for example, can add to unsymmetrical dipolarophiles to give two diastereomers. 

Although the allylation reaction is formally analogous to the addition of allylic boranes to carbonyl derivatives, it does not normally occur through a cyclic TS. This is because, in contrast to the boranes, the silicon in allylic silanes has little Lewis acid character and does not coordinate at the carbonyl oxygen. The stereochemistry of addition of allylic silanes to carbonyl compounds is consistent with an acyclic TS. The -stereoisomer of 2-butenyl(trimethyl)silane gives nearly exclusively the product in... 

Entry 23 was part of a synthesis of the pancratistatin structure. The lactone ring was used to control the stereochemistry at the cyclization center. Noncyclic analogs gave a mixture of stereoisomers at this center. In this reaction, triphenylstannane gave much better yields than tri-n-butylstannane. 

At least two derivatives of pyran have been used for the primary synthesis of quinoxalines. Thus o-phenylenediamine (390) and 6-(p-methoxyphenyl)-6-methyl-5,6-dihydro-2//-pyran-2,5-dione (391) in methylene chloride at 20°C open to the air for 48 h gave 3-[2-hydroxy-2-(p-methoxyphenyl)propionyl]methyl-3,4-dihydro-2(1 //)-quinoxalinone (392) (as a mixture of two stereoisomers) in 92% yield 488 the 3,4,4a,5,6,7,8,8a-octahydro analog was made similarly from 1,2-diaminocyclohex-ane (91% of two separable stereoisomers).488... 

The oxaspirocyclization was applied to the synthesis of theaspirone and vitispirane (equations 26 and 27)59. Under slightly modified reaction conditions where water is employed as the major solvent, palladium-catalyzed 1,4-oxidation of 64 afforded 65. Alcohol 65 was oxidized to theaspirone, which was obtained as a 1 1 isomeric mixture of cis and trans isomers. When the analogous reaction was performed at a lower pH by the use of trifluoroacetic acid, vitispirane was formed in high yield, again as a 1 1 isomeric mixture of stereoisomers. 

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