Rings fused conformations

Polycyclic compounds are common in nature, and many valuable substances have fused-ring structures. For example, steroids, such as the male hormone testosterone, have 3 six-membered rings and 1 five-membered ring fused together. Although steroids look complicated compared with cyclohexane or decalin, the same principles that apply to the conformational analysis of simple cyclohexane lings apply equally well (and often better) to steroids. 

P2j Z = 2 Dx = 1.33 R = 0.09 for 792 intensities. The molecule is tricyclic, with a five-membered and a seven-membered ring fused to the ribofuranose ring. Both five-membered rings have an envelope conformation. The seven-membered ring has the same conformation as is found in 2,5-arabinosylcytidine monophosphate.79 ... 

Ci9H2408Ss Ethyl 3,7-anhydro-6,8-0-benzylidene-4-deoxy-2-(ethyl-enedithio)-D-talo-2-octulosonate (ABTOET)265 P212121 Z = 4 Dx = 1.359 R = 0.045 for 1,520 intensities. The structure contains a fused pyranoid ring and a dioxolane ring. The conformation of the pyranoid part is a distorted 6C3, with Q = 57 pm, 6= 20°,

chair conformations of the dioxolane and the dithiene rings... 

Rate constants for reaction of Ca2+aq with macrocycles and with cryptands (281,282,291) reflect the need for conformational changes, considerably more difficult for cryptands than for crown ethers, which may be considerably slower than formation of the first Ca2+-ligand bond. Ca2+aq reacts with crown ethers such as 18-crown-6 with rate constants of the order of 5 x 107M 1 s, with diaza crown ethers more slowly (286,326). The more demanding cryptands complex Ca2+ more slowly than crown ethers (kfslow reaction for cryptands with benzene rings fused to the macrocycle. The dominance of kA over kt in determining stability constants is well illustrated by the cryptates included in Table X. Whereas for formation of the [2,1,1], [2,2,1], and [2,2,2] cryptates kf values increase in order smoothly and gently, the k( sequence Ca[2,l,l]2+ Ca[2,2,l]2+ Ca[2,2,2]2+ determines the very marked preference of Ca2+ for the cryptand [2,2,1] (290). 

X-Ray elucidation provided a final proof for the structure elucidation of 8-(4-chlorophenyl)-8-hydroxy-5-methyl-8/f-[l,4]thiazino[3,4-H[l,2,4]oxadiazol-3-one 49, which was obtained as a product of ring-transformation reaction <1997J(P2)2407>. This analysis revealed that the oxadiazole ring is planar, whereas the thiazine ring is in a distorted half-chair conformation with a displacement asymmetry parameter AC2 (S—G(3)) = 0.031. The structure analysis represents the first X-ray elucidation of a [l,4]thiazine ring fused to a [l,2,4]triazole moiety. 

Of the six possible conformers containing chair rings, two tra/j5-fused conformers 18 and 19, with a slight excess of 18, could be identified as major conformers, with one a>-fused conformer as a minor constituent in the C NMR spectrum of l-methylperhydropyrido[l,2-h]pyridazine in acetone-dfi in the temperature range -75 to -89°C (78JA4012). The low intensity of the signal of the c -fused conformer did not allow determination of the exact structure of this component. The results of low-temperature cyclic voltammetry experiments supported the NMR findings. 

The racemic form of methyl 3,4-anhydro-/3-mycaroside (82) has been synthesized 35 it is an epoxide of a branched-chain sugar having the oxirane ring fused to the pyranose ring. Hydrolysis of the oxirane ring with hydrochloric acid in aqueous p-dioxane occurs with substitution at C-4, the (more accessible) secondary position, as would be expected from its favored °H, conformation (82) anticipated, because it is an analog of the 3,4-anhydro-D-allopyranoside derivatives considered previously (see p. 148). 

The nomenclature used in this section differs from the strictly systematic rules used by Chemical Abstracts to index fused-ring azetidinones, but conforms more to that commonly found in the literature describing the /3-lactam antibiotics. A description of this naming system is given in the introduction (Section 5.12.1). Virtually all of the compounds discussed in this section contain a carboxyl group adjacent to the /3-lactam ring and conform to general structure (49). 

Nonbenzenoids These compounds generally have two or more rings fused together, but none of the rings is a benzene structure, and they conform to HiickeTs rule, i.e. they have (4u + 2) tt electrons, and are aromatic compounds, e.g. azulene. 

Ring-A methyl-substituted perhydropyrido[l,2-c][l,3]oxazines carrying axial methyl groups in the trans-fused conformation (273 R1 = H, R2 = Me and 273 R1 = Me, R2 = H) all adopt a position of conformational equilibrium similar to 271 272.135... 

Of the four diastereomeric 8,9,10,11,1 la,1 lb,12,13-octahydro-7a//-quino-[l,2-c][l,3]benzoxazines (304), three prefer the trans-fused conformations 305-307. The overlap of the nitrogen lone-pair orbital with the aromatic ring affects the Jgem and Aae values of the NCH20 protons in these isomers (Sections II,B,1 and 2), and the Bohlmann absorption (Section II,E,1) is no... 

A -NMR study137 of the conformational equilibria in syn- and anti-perhydrodipyrido[l,2-c 2, l -/]pyrimidine indicates the existence of these exclusively in the conformations 347 and 348. Substitution in 347 by axial methyl groups in the end rings shifts the equilibria somewhat toward the cis-fused conformation. Thus the cis-( 1 -H, 12a-H)-1 -methyl derivative adopts an equilibrium containing 85% of the trans-syn-trans conformation 349 and 15% of the trans-syn-cis conformation 350. Similarly, the 1-methyl analog in the anti series exists as 70% trans-anti-cis 351 with 30% cis-anti-cis 352.137... 

The tetrahydroprotoberberines may exist in solution as an equilibrium mixture of the trans-fused conformation [75] and two m-fused conformations [76] and [77] interconvertible by nitrogen inversion [75] [76] and by ring inversion [76] [77]. 

Anhydro sugars of this type, having an oxirane ring fused with a pyranose or furanose, possess 3,7-dioxabicyclo[4.1.0]heptane (67) and 3,6-dioxa-bicyclo[3.1.0]hexane (68) basic skeletons, respectively. 2,3-Anhydro-pyranoses usually adopt 5HQ or °HS and 3,4-anhydropyranoses adopt 1H0 and °H conformations in the ground state. 

When we come to two six-membered rings fused together 33, it is not so obvious that there is an inside and an outside, but if both rings have the chair conformation 33a there is an inside and an outside. Even if there is only one ring-junction substituent 34 there is still a distinction. 

These compounds consist of two or more benzene rings fused together, and the number of delocalized tt electrons conforms to the HUckel (An + 2) rule for aromaticity. Figure 14.2.2(a) shows the carbon skeletons of some typical examples. 

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