And spiranes

The property of chirality is determined by overall molecular topology, and there are many molecules that are chiral even though they do not possess an asymmetrically substituted atom. The examples in Scheme 2.2 include allenes (entries 1 and 2) and spiranes (entries 7 and 8). Entries 3 and 4 are examples of separable chiral atropisomers in which the barrier to rotation results from steric restriction of rotation of the bond between the aiyl rings. The chirality of -cyclooctene and Z, -cyclooctadiene is also dependent on restricted rotation. Manipulation of a molecular model will illustrate that each of these molecules can be converted into its enantiomer by a rotational process by which the ring is turned inside-out. ... 

Oleandomycin, its ester (triacetyloleandomycin) and spiran rdn have a similar range of activity as erythromycin but are less active. Resistance develops only slowly in chnical practice. However, cross-resistance may occur between all four members of this group. 

The synthesis of propellanes and spirans is reviewed, attempting to explain why a starting material sometimes yields a member of one class or the other, but apparently not a mixture of both. 

In addition to this nonequivalence, the imprecise definition of the chirality axis and difficulties in dealing with stereogenic centers in polycyclic structures led to confusion in the case of certain adamantanes, cyclohexanes and spiranes (see Section 1.1.5.3.3.). These problems were solved in the late 1970s and it was recommended1 that the PjM rather than the obsolete aR/aS description be used. This use is recommended as it leads to a considerable simplification of static Stereochemistry. 

The point groups V d have, in addition to the axes defining Vn, n diagonal planes Od which bisect the angles between successive twofold axes. The ad and C2 axes imply that there is also an S2n and, if n is odd, a center of symmetry Molecules of T>2d symmetry have the shape of two equivalent halves twisted b 90°, for example, allene and spiran shown here ... 

Molecules that do not possess an asymmetric center may still have nonsuperimposable mirror images and exist as enantiomers. These molecules contain a chiral plane or chiral axis and are dissymmetric with respect to either that plane or axis. The structures of the enantiomers of the sedative-hypnotic methaqualone are presented in Fig. 4. In this molecule there is a chiral axis between the nitrogen atom (N-1) and phenyl ring (C-1). The dissymmetry of the two forms of the molecule is a result of hindered rotation around this axis, which is due to steric interactions between methyl groups (M-1 and M-2). Other axially dissymmetric molecules include allene, biaryls, alkylidenecyclohexanes, and spiranes. Planar dissymmetric molecules are exemplified by molecules such as tra s-cycloalkenes. 

Part 2 Tin Tetraorganyls R3SnR (1975). Part 3 Tin Tetraorganyls R2SnR 2, R2SnR R, RR SnRR, Heterocyclics and Spiranes (1976). Part 4 Organotin Hydrides... 

Carbenoid insertions. Advantage has been taken of the insertion reaction in the formation of dehydropeptides, a-silyl a-amino acid derivatives, functionalized tetrahydrofuranones,- 1,4-dioxenes, and spirane systems. ... 

Chirality may exist in many molecules that do not possess a chiral center. Such compounds may possess a chiral plane or a chiral axis, and are said to be dissymetric with respect to either that plane or that axis. Certain optically active allenes, biaryls, alkylidenecyclohexanes, and spiranes provide examples of axially dissymmetric molecules (chiral axis), irons-Cycloalkenes exemplify planar dissymmetry in molecules. The configurations of these classes may be specified by the Cahn-Ingold-Prelog convention using the usual R and 5 descriptors. Special subrules, which we will not describe here, are applied to this purpose. The interested reader is referred to references 8 (see p. 43) and 9 for details. Scheme 2.1 presents some molecules that are optically active because of planar or axial dissymmetry, and for which the absolute configurations have been determined. 

Interestingly, the arborols possessing spirane or biphenyl moieties did not induce gelation in an aqueous environment. Newkome and co-workers concluded that the biphenyl and spirane spacer units were more sterically demanding than the related alkane series, which disrupted the molecular in-... 

A recent novel synthesis of desacetamidoisocolchicine (211) employs the cyclopropyl-ketone (205) as a functionalized tropolone equivalent. Acid treatment of the acetal (207), formed by condensation of (205) with the Grignard reagent (206), led to rearrangement, via dienone (208) and spiran (209), to the tricyclic system (210). Oxidation of (210) then gave the isocolchicine (211). 

Minkin, V. and Komissarov, V.N., Perimidinespiroq clohexadienones — a novel photo- and thermchromic system. Mol. Cryst. Liq. Cryst, 297, 205, 1997 Metelitsa, A.V., Komissarov, V.N., Knyazhansky, M.I., and Minkin, V.I., New photochromic bisspiroq clic systems. Mol. Cryst. Liq. Cryst., 297, 205, 1997 Kharlanov, V.A., Effect of molecular oxygen on the kinetics of dark transformation of quinonimine structure and spiran structure for photochromic spirans of perimidine series, Zh. Org. Khim., 35, 631, 1999. 

Compounds containing nitrogen atom that are capable of forming four bonds and having different substituents may be optically active. In such compounds, there is a certain analogy to a chiral tetrahedral carbon atom. This type of compounds includes ammonium salts, amine oxides, and spiranes where nitrogen atom is a central one (Figure 2.26). 

---