Synclinal

Folds are features related to compressional, ductile deformation (Fig. 5.10). They form some of the largest reservoir structures known. A fold pair consists of anticline and syncline. 

The CH fragment which is linked to the OH group (Sh = 5.45 ) can easily be located in the H and NMR spectra. The chemical shift values Sc =74.2 for C and Sh = 3.16 for //are read from the CH COSY plot. The H signal at S,i = 3.16 splits into a triplet (11.0 Hz) of doublets (4.0 Hz). The fact that an antiperiplanar coupling of 11 Hz appears twice indicates the diequatorial configuration (trans) of the two substituents on the cyclohexane ring 5. If the substituents were positioned equatorial-axial as in 4 or 5, then a synclinal coupling of ca 4 Hz would be observed two or three times. 

The synclinal conformation (sc) is appropriate for overlap of an oxygen nonbonded pair with the a C—Cl orbital. The preferred ap relationship, requires an antiperiplanar alignment of a lone-pair orbital with the bond to the electronegative substituent. Because of the donor-acceptor nature of the interaction it is enhanced in the order F < O < N for the donor (D) atom and N < O < F for the acceptor (A) atom. 

It has been observed that the best neutral cation carriers have the synclinal arrangement of binding atoms in the backbone (see 7.2). Syntheses of both an aliphatic and aromatic ligand are shown below in Eqs. (7.12) and (7.13). 

The Michael addition of enamines to nitroalkenes proceeds with high Yyn selectivity. The Yyn selectivity is explained by an acyclic synclinal model, in which there is some favorable interaction between the nitro group and the nitrogen lone pair of the enamine group CEq. 4.67i. Both Z- and E-nitrostyrenes afford the same product in over 90% diastereoselecdvity. 

The diastereoselectivity of these reactions was initially explained in terms of acyclic, an-tiperiplanar transition states23, although a synclinal process has since been proposed on the basis of the stereoselectivity of Lewis acid induced intramolecular allylsilane- aldehyde cycliza-tions56. 

An open-chain antiperiplanar transition state was initially proposed for this reaction74, although a synclinal alternative has since been suggested56. 

The stereoselectivity of the boron trifluoride induced reactions was initially discussed in terms of open-chain, antiperiplanar transition states66. However studies of Lewis acid induced intramolecular allylstannane-aldehyde reactions are supportive of a synclinal process56,67. 

The stereoselectivity of these intermolecular reactions between 1-alkoxyallylstannanes and aldehydes induced by boron trifluoride-diethyl ether complex is consistent with an open-chain, antiperiplanar transition state. However, for intramolecular reactions, this transition state is inaccessible, and either (Z)-.yyn-products are formed, possibly from a synclinal process105, or 1,3-isomerization competes113. Remote substituents can influence the stereoselectivity of the intramolecular reaction114. 

The selectivity observed can be explained by a synclinal transition state in which the Lewis acid is chelated to two acceptor groups32. 

The diastereoselectivity observed can be explained by a synclinal transition state, probably influenced by chelation and/or stereoelectronic effects of the developing cation38. The minor product is formed via an antiperiplanar transition state. All compounds obtained are useful precursors for several spirocyclic natural products, such as terpenes like lubimine or acoradi-ene. 

A resident stereocenter in the enone part can control the formation of two new stereocenters in one step, guided by a synclinal transition state and an axial cyclization mode. The major product on cyclization of 4-methyl-3-[6-(trimethylsilyl)-4-(Z)-hexenyl]-2-cyclohexenone was formed in a ratio of 7.5 13S. 

In the Lewis acid mediated reaction the developing carbenium ion in C is stabilized by the nearby 7t-electrons of the titanium or aluminum enolate. This generates as the major diastereomer the 3,3a-/r .v-relationship between the substitution at the ring junction and the vinyl group at C-3 via a synclinal transition state. 

Zirconocene dichloride 121 derived from (l-phenylethyl)cyclopentadienyl ligand is formed as a mixture of diastereomers from which the racemic form can be isolated by fractional crystallization. This complex was studied by X-ray diffraction methods and revealed a virtually chiral C2-symmetrical conformation in which the chiral ring-substituents are arranged in a synclinal position relative to the five-membered ring. It was proposed that this conformation is preserved in solution. Using 121 as catalyst the influence of double stereodifferentiation during isospecific polymerization of propylene (Eq. 32) was demonstrated for the first time [142],... 

Proton resonances for aU residues were assigned using a combination of COSY, TOCSY, HSQC, and HMBC experiments. The large values of y(NH/H-C(/9)) and the small values of J(H-C(a)/H-C(y9) were indicative of antiperiplanar and synclinal arrangements respectively, around those bonds. In addition, medium-range NOE connectivities H-C(/ )i/NH +i, H-C(a)i/NH, + i, NHi/NH +i were consistent... 

Peptides built from y-amino acids with L-amino acid-derived chirahty centers form a right-handed (P)-2.6i4 hehx of ca. 5 A pitch with both ethane bonds in a -y)-synclinal conformation (mean values for dihedral angles 9 and 82 of central residues 2-5 in compounds 141 are 72.5 and 64.3°, respectively Fig. 2.36 A and B Tab. 2.8). 

Fig. 2.47 The (P)-2.5-helical structure of N.N -linked oligoureas as determined by NMR meaurements in pyridine-c/5. (A) Stereo-view along the helix axis of a low energy conformer of nonamer 178 generated by restrained molecular dynamics calculations. (Adapted from [274]). The helix is characterized by (i) a rigid +)-SYnclinal arrangement around the C(a)-...

The observed selectivities are consistent with a model introduced by Danishefsky [53], which invokes a synclinal approach of the crotylsilane with the crotyl methyl group pointed away from the ring (Fig. 5). In the case of compound 148, such an approach results in a 5jyn-pentane interaction between the crotyl methyl group and the equatorial methyl group, leading to the turnover in selectivity that was observed. 

Fig. 5. Synclinal approach of crotyl silanes onto oxocarbenium ions...

Both anti-synclinal and anft -periplanar TSs are considered to be feasible. These differ in the relative orientation of the C=C and C=0 bonds. The anti-synclinal arrangement is usually preferred.94... 

The addition reaction of allylsilane to acetaldehyde with BF3 as the Lewis acid has been modeled computationally.95 The lowest-energy TSs found, which are shown in Figure 9.2, were of the synclinal type, with dihedral angles near 60°. Although the structures are acyclic, there is an apparent electrostatic attraction between the fluorine and the silicon that imparts some cyclic character to the TS. Both anti and syn structures were of comparable energy for the model. However, steric effects that arise by replacement of hydrogen on silicon with methyl are likely to favor the anti TS. 

Fig. 9.2. Most favorable transition structures for reaction of allylsilane with acetaldehyde-fluoroborane (left) anti synclinal (right) syn synclinal. Reproduced from J. Am. Chem. Soc., 119, 12131 (1997), by permission of the American Chemical Society.

Intramolecular reactions can also occur between carbonyl groups and allylic silanes. These reactions frequently show good stereoselectivity. For example, 7 cyclizes primarily to 8 with 4% of 9 as a by-product. The two other possible stereoisomers are not observed.98 The stereoselectivity is attributed to a preference for TS 7A over TS 7B. These are both synclinal structures but differ stereoelectronically. In 7A, the electron flow is approximately anti parallel, whereas in 7B it is skewed. It was suggested that this difference may be the origin of the stereoselectivity. 

Synclinal and antiperiplanar conformations of the TS are possible. The two TSs are believed to be close in energy and either may be involved in individual systems. An electronic tt interaction between the stannane HOMO and the carbonyl LUMO, as well as polar effects appear to favor the synclinal TS and can overcome the unfavorable steric effects.161bi 162 Generally the synclinal TS seems to be preferred for intramolecular reactions. The steric effects that favor the antiperiplanar TS are not present in intramolecular reactions, since the aldehyde and the stannane substituents are then part of the intramolecular linkage. 

In the synthesis shown in Scheme 13.15, racemates of both erythro- and threo-juvabione were synthesized by parallel routes. The isomeric intermediates were obtained in greater than 10 1 selectivity by choice of the E- or Z-silanes used for conjugate addition to cyclohexenone (Michael-Mukaiyama reaction). Further optimization of the stereoselectivity was achieved by the choice of the silyl substituents. The observed stereoselectivity is consistent with synclinal TSs for the addition of the crotyl silane reagents. 

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