Anti-endo conformations

Figure 3. Linalyl derivatives cyclize preferentially from the anti-endo conformation.

The pinene cyclases convert the anomalous linalyl enantiomer to abnormal levels of acyclic (e.g. myrcene) and monocyclic (e.g. limonene) terpenes, these aberrant products perhaps arising via ionization of the tertiary substrate in the transoid or other partially extended (exo) form (see below). In any event, for all "normal" cyclizations examined thus far, the configuration of the cyclizlng linalyl Intermediate has been confirmed to be that which would be expected on the basis of an anti-endo conformation. Scattered attempts at intercalating the cyclization cascade with analogs of proposed cyclic intermediates (e.g. a-terpinyl and 2-pinyl pyrophosphate) have been unsuccessful (20,35,36). 

Figure 3.11 shows the biosynthesis of (+)-bornyl pyrophosphate (the precursor of (-l-)-borneol (28) and (+)-camphor (29)) and of (+)-sabinene (30) (the precursor of the thujones) from 3R)-20 cyclized in an anti,endo conformation (Wise et al. 1998). Other related products include camphene (31) and 1,8-cineole (32). The chemistry involved in the formation of the final products includes Wagner-Meerwein rearrangements of hydride and (in the case of camphene) a skeletal carbon-carbon bond as well as simple cyclizations. The biosynthesis of (-)-a-and P-pinene (33 and 34) proceeds along similar lines from (3S)-20 and is shown in Figure 3.12. 

Incisive model studies by Arlgonl and co-workers have, in fact, demonstrated that the anti-endo conformation of the linalyl system is preferred in the cyclization to a-terpineol, This, along with the confirmation that allylic transposition reactions occur by a net suprafaclal (syn) process,allows deduction of the helical folding of geranyl pyrophosphate and thus, by summation, all of the basic stereochemical elements of the coupled isomerlzatlon-cyclization. 

Figure 10.21 Relationship of deoxyribose and the guanine base in B- and Z-DNA. Rotation to affect either the syn or the anti conformations is done about the N-glycoside bond, as shown by the curved arrow. Also note that in Z-DNA, the 3 carbon is above the plane of the sugar, whereas in B-DNA, the 2 carbon is. This is the endo conformation. (Reproduced by permission from Rich A, Nordheim A, Wang AHJ. The chemistry and biology of left-handed Z-DNA. Annu Rev Biochem 53 701-846, 1984.)...

In contrast to the tricarbonyl halide complexes, it is the exo conformation of () -allyl)FeCp(CO) complexes (163) which is preferred, and at equilibrium only traces of the endo isomer can be observed. Existing crystal structures of members of this class are of the exo conformation.Reasonable percentages of the endo conformer are realized upon low-temperature photolyses of their () -allyl)Fp precursors, however, and the activation barrier to endolexo conversion has been determined to be AG = 100.8kJmol for the 2-methallyl complex. In addition, the anti isomers, which can be prepared stereospecifically from the (j7 -allyl)Fp complexes, isomerize to the syn isomers with brief heating. The Fe NMR spectra are known for several of the complexes where CO has been replaced by a phosphine or phosphite (145) the chemical shift is highly dependent on the nature of the phosphorus ligand. ... 

In general, the 3 -endo conformation of fluoronucleoside is essential for antiviral activity against RNA viruses although the anti-HIV activity is an exception (the details of anti-HIV activity are addressed in the next section). 

Figure 1.75 Z-form DNA results from wholesale conformational changes in deoxyguanosine nucleotide (dpG) residues. The more usual anti, +sc, C2 endo nucleotide conformation gives way to a syn, ap, C3 endo conformation, (a) Illustration of syn dpG conformation, (b) Ball and stick depiction of pdG(syn, ap, C3 endoj.pdC base pair as found in Z-form DNA.

Figure 9 The 785-797 loop of I6S ribosomal RNA. (a) Secondary structure deduced from NMR data and a spanning tree. The arrow.s indicate the selected edges in the spanning tree, (b) MC-SYM. script corresponding to the spanning tree in (a). The ANYJ NT list contains combinations of C3 -, Clf-endo and anti nucleotide conformations...

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