Stereoisomers with rings

When 34d was generated in the presence of enol ethers 190, [4 + 2] cycloadducts 191a-c were isolated [47] (Table 17). The cycloadducts 191b and 191c proved to be pure stereoisomers, with the two heterocyclic ring systems cis connected. 

Recent evidence for the formation of hemiketal intermediates iqmn acylation of alkynides has been obtained from glucopyranolactones. Treatment of tetrabenzyl (55) with the anion from l-benzyloxy-3-bu-tyne gave a quantitative yield of hemiketal (56) which showed IR absorptions for the OH and alkyne portions of the molecule (X = 3350 cm" and 2250 cnr, respectively). This compound was stereospecifi-cally reduced to the C-glycoside (57) with triethylsilane/BFs-etherate (overall yield 72% Scheme 18). None of the other stereoisomer or ring-opened product was obtained. ... 

The air-stable enantiomers of the As2S2 macrocycle XIX have been obtained stereoselectively via a Pdn-assisted cyclization reaction as illustrated in Scheme 12. Demetallation is via treatment with CN-. The use of racemic precursors has been investigated and quantitative interconversion of stereoisomers is detailed in this work, together with ring contraction to give the 7-membered ring As/S ring compound XX.44... 

Molecules with the same connectivity but with some of their atoms arranged differently in space —> Geometrical isomers are stereoisomers with different arrangements in space on either side of a double bond, or above and below the ring of a cycloalkane. 

The regioselectivity of LEH has been studied experimentally with different substrates. Studies with the enantiomers of 1-methylcyclohexene oxide (1 and 2, Scheme 2) revealed preferred attack at the methyl-substituted oxirane carbon, Cl, with a regioselectivity of 85(C1) 15(C2). This indicated an acid-catalyzed mechanism, which would result in preferred attack at the more substituted carbon. However, conversion of limonene-1,2-epoxide did not support this conclusion and showed somewhat intriguing results. Exclusive attack at the more substituted carbon (Cl) is seen for the stereoisomers 4 and 5, while exclusive attack at the less substituted carbon (C2) is observed for stereoisomers 3 and 6 (Scheme 2). Interestingly, the two limonene-1,2-epoxide stereoisomers with the same stereochemistry at the oxirane ring, (IR,2S) for 3 and 5 and IS,2R) for 4 and 6, exhibit attack at opposite carbons (Scheme 2). A suggested explanation for the differences was differential binding of the substrates in the active site, which would lead to attack at different carbons. ... 

Assignments have been accomplished for four structural isomers and stereoisomers with pyrim-ido[l,6-c][l,3]benzoxazine and pyrimido[l,2-c][l,3]benzoxazine structures <75S802>. The [l,4]oxa-zino[3,4-Z ]quinazoline structure (216), rather than a possible seven-membered ring structure, was established for the cyclization product of (305 R = CH2C02Me, = H) (see Section 8.25.3.6.8)... 

About 60 glycosides of S-rhodomycinone (39f) and other stereoisomers with sugar residues at C-7, C-10, or in both positions have been reported (see Table 5). C-substitution in ring A is scarcely observed in anthracy-cHnes exceptions are mutactimycin A (40a) and D (40b) from a Strepto-... 

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