Half-boat

Syntlietic cliemists can now work witli tlitee-dlmensional pictures of tlie conjugate addition available on a website [80]. In tlie absence of steric hindrance i5-nietliylcydobexenone, for example), an "axial attack" tlitougb a balf-diait conformation is favored, while in tlie corLisone syntliesis an "equatorial attack" tliroLigli a half-boat conformation is favored because of tlie constraint imposed by tlie bicydic tings [83]. 

Conjugated chains, 14, 46 Correlation diagrams, 44, 50 Cyclobutadiene, 171 Cyclobutane, 47, 222 orbital ordering, 26 through-space interactions, 26 Walsh orbitals, 27 Cyclobutene, 200 Cyclohexane, 278 Cyclohexene (half-boat), 274 Cyclopen tadiene, 225 Cvclopen tadienone, 269 Cyclopentadienyl anion, 237 Cyclopentane, 254 Cyclopen ten e, 241 Cyclopropane, 41, 47, 153 construction of orbitals, 19, 22 Walsh orbitals, 22, 36, 37 Cyclopropanone, 48, 197 bond lengths, 38 Cyclopropen e, 49, 132 reactivity, 40... 

We also performed a single-crystal X-ray structure analysis of this lead compound. The solid state structure of this compound depicted in Fig. 3-15 shows a half-boat-like ( sofa ) conformation with the 9-phenanthryl group in a quasi-axial or r/Mf/.v/-flagpole position, and the a, 3-unsaturated exocyclic ester in a s-cis conformation. This cleft-like conformation is advantageous for the creation of centers with a high recognition ability, since one enantiomer fits in better than the other thus leading to selectivity. 

In six-membered cyclic nitronates (e.g., in (85)) adopting a half-chair conformation, the C-6 and C-5 atoms deviate from the plane of four atoms in the opposite directions, the deviation of the C-6 atom being substantially larger (the deviation of the C-5 atom. The latter is generally at most 0.05 A). It should be noted that one six-membered cyclic nitronate adopts a half-boat conformation (data from the Cambridge Structural Database, see also Fig. 3.2 and its discussion). 

The conformation B (Fig. 3.2) is a half-boat, in which the C-5 and C-6 atoms deviate in the same direction from the plane of the nitronate fragment but at different angles (01 =53.7° and 02 = —46.6°, that is, A = 7.1°). The discussion of the conformation A is valid for the conformation B. Interestingly, X-ray data for one six-membered cyclic nitronate adopting the conformation B are available in the Cambridge Structural Database. 

Pyranose Conformations. Figure 3 shows the different conformations for 6-membered rings (adapted from a drawing by Jeffrey and Yates (27)). There is a 9 parameter besides Q and ( > because several types of puckering are possible for a given Q and < ). In addition to the E (envelope) notation used in Figure 3, six-membered rings with only one out-of-plane atom are also called sofas or half-boats. The E descriptor was selected here because S is already used to denote skewed pyranose conformations (which have two atoms on opposite sides of the plane, separated by one atom). The H label is already used for half-chairs, which have two adjacent atoms on opposite sides of the mean plane. Typically, the E and H forms are not iiqportant unless a double bond is present. 

Figure 3. The conformational sphere for pyranoid rings. The perfect chairs are at the north and south poles (0=0 and 180 , respectively). The boat and skew (B and S designations) at the equator permit pseudorotation that is slightly hindered, at least for cyclohexane. The envelopes, E (also called sofas and half-boats), and half-chairs, H, are not observed for rings coiqposed of saturated carbon and oxygen atoms, but are iiqportant forms for rings with unsaturated carbon atoms. The aiqplitude of puckering corresponds to the radius of the sphere.

Figure 9a. Spherical polar depiction of pyranose puckering. The equatorial belt is the path of facile pseudorotation through all the Boats and Skew-boats rotation). Perfect chairs are at the North and South poles, and the Half-boat (Envelope) forms are at 6 of 54.7. This diagram has a reversed direction of positive pseudorotation from that shown in figure 3, Chapter 1.

Both steric repulsion and anomeric effects proved to influence the conformational equilibria of 5,6-dihydro-4//-1,3-oxazines involving half-boat structures. For the /ra t-4,6-dialkyl-substituted compound 84, the conformational equilibrium was driven by steric repulsion it appeared that a 6-alkyl group in the axial position 84a is more hindered than an axial 4-alkyl 84b. However, for the /ra t, 6-diaryl-substituted derivative 85, the major conformation in the... 

Fully conjugated 1,2-thiazines have been prepared with both S(ll) and S(vi) oxidation states. While the zwitterionic compound 11 has been known for some time <1984CHECI(3)995, 1996CHEC-II(6)349>, thiazinylium salts 12 have only recently been prepared. Both fully unsaturated 1,2-thiazine derivatives are considered to be nonaromatic due to poor p-d Jt-bonding. Eurthermore, the six-membered ring of l-alkyl-l,2-thiazine 1-oxide 11 is not planar, but instead exists in a puckered, half-boat conformation thereby precluding aromaticity <1978CC197>. 

The variable valency of sulfur allows the formation of S-oxides (11) which unlike thiabenzene 1-oxides are non-aromatic and exist in half-boat conformations (78CC197). On the other hand, acid treatment of 6-hydroxy-6H- 1,2-oxazines (12) yields cations (13) which are analogous to pyrylium salts (74ZOR1513). 

A half-boat conformation of the cyclobutanone ring (in an octant projection) of 74 correctly explains the observed positive CD near 300 nm. A pseudo-axial chlorine substituent in 75 substantially enhances the positive Cotton effect. The cyclobutanone ring in 76 is nearly planar, thus the effect of exo and endo chloro substituents is effectively cancelled while the remainder of cholestane skeleton resides in a negative octant175. 

In recent studies, acceptable yields of (3-configurated 2-deoxy glycosides by application of the NIS reaction to uronic ester glycals like 38 have been reported [19]. By NMR studies the equilibrium conformation of the glycal 38 is shown to be shifted towards the inverted half-chair or half-boat conformation [5H4(D)]. 

Figure 8.9. One octant of a sphere on which the conformations of cyclohexane can be mapped. Special conformations C, chair B, boat TB, twist boat HB, half-boat and HC, half-chair. (From Cremer and Pople [1975a,b].)...

The value of q3 = (6) V2R (R is the CC bond length) is 0.63 A. Under pseudorotation the equatorial boat-shaped structures B (0 = 90°, = 0, 60°, 120°,.. . ) turn into a twist-boat structure TB (0 = 90°, = 30°, 90°,.. . ). The transitions between the chair and twist boat structures involve the intermediate formation of half boat (HB) and half chair (HC) structures. Quantum chemical calculations carried out by Dixon and Komornicki [1990] show that the axial structure C with symmetry D3d is stable. The energies of structures B and TB are 7.9 and 6.8kcal/mol higher than C. The barrier for transition from C to TB is 12.2-12.4 kcal/ mol. Because of the high barriers for pseudorotation, only thermally activated conformational transitions occur in cyclohexane. 

The central ring of compound (11) adopts a fixed boat or half-boat conformation and the proton at lower magnetic field occupies a (pseudo) equatorial position and that at higher field a (pseudo)axial position consequently, the molecule adopts a bent conformation <90CL1197>. 

Next, other unsaturated carbocyclic systems are discussed, such as dibenzo-cyclo-heptadienes ( atropisomeric biphenyls), dihydropleiadenes, cycloheptatrienes and their benzo-derivatives. Most of these compounds exist in a half-boat or boat conformation. Tropones are considered in the light of recently published work. 

The X-ray structure analysis110 of 64 shows that the (Z,Z, )-enaminodione moiety is strictly planar, and that the cyclohexane-1,3-dione ring adopts a half-boat conformation, with Cx—C4 and C6 in the plane and C5 above it. 

In a review of this work written about the same time, Shoppee ) changes his wording slightly to imply a half boat structure 2 as the transition state for interconversion. 

Schmid, and his collaborators i ) have made the point that there are several conformations similar to the half chair and half boat of compcirable energies. 

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