Saturated six-membered ring

When immediately followed by -in or -ine, phospha- should be replaced by phosphor-, arsa- by arsen-, and stiba-by antimon-. The saturated six-membered rings corresponding to phosphorin and arsenin are named phosphorinane and arsenane. A further exception is the replacement of borin by borinane. 

Of the legion of partially saturated six-membered ring heterocycles, an idea of their stability, or lack of it, can normally be gained by consideration of the thermodynamic stability of the various components which can be identified in them. Thus, those rings which contain ester or amide functions can be expected to possess the chemical reactivity and the... 

A large amount of data are available on the C spectra of saturated six-membered ring systems. The subject has been reviewed in detail by Eliel and Pietrusziewicz (79MI20101). 

Table 7 C Chemical Shifts S, p.p.m. from TMS) in Saturated Six-membered Rings (4S) ...

When two heteroatoms are present in a saturated six-membered ring their effects are approximately additive. Apart from the case of two a oxygen atoms—in 1,3-dioxane (48) the shift of C-2 is S 95.4 instead of 5 108 which a double shift of 40 p.p.m. would require—predictions of shift made on this basis are usually accurate to within 5 p.p.m. and are generally much closer than this. Observed shifts for a few representative examples are shown in structures (48)-(52). 

Strychnine, the most celebrated member of the Strychnos alkaloids, possesses a complex polycyclic structure which is assembled from only twenty-four skeletal atoms. In addition to its obvious architectural complexity, strychnine s structure contains a contiguous array of six unsymmetrically substituted tetrahedral (asymmetric) carbon atoms of which five are included within one saturated six-membered ring. The intimidating structure of the strychnine molecule elicited the following remark by Sir Robert Robinson in 1952 For its molecular size it is the most complex substance known. 5... 

N.m.r. spectroscopy has played an important part in determining the stereochemistry of the 1,3-dioxaphosphorinanes (52). The presence of the saturated six-membered ring means that there are usually conformational effects to be unravelled before configurational assignments can be made. The chair conformation is generally dominant. Phosphorus substituents which exhibit shielding effects show that in many P " phosphorinanes this substituent occupies an axial position and Sis( has been used to establish the equatorial conformation of a t-butyl substituent at C(5). Even in P" derivatives the isomer possessing the bulkiest P-substituent in an axial... 

Synthesis of the heterocyclic core possessing a fully saturated six-membered ring can be achieved using a catalytic hydrogenation (see Section 11.10.4.5.5). The six-membered ring can also be formed starting from the appropriate... 

Tetrahydrotetrazolo[l,5- ]pyridine 52 was reacted with dimethyl sulfate to give a mixture of quaternary salts 1-methyl 53 and 2-methyl compounds 54 from which the 1-alkyl compound was separated as a crystalline hexafluorophosphate salt 53 (A = PF6) in good yield. This salt when treated with potassium hydride in the presence of 18-crown-6 and KCN underwent deprotonation to give the saturated six-membered ring 55. 

Glowka et al. studied two tetrahydro[l,2,4]triazolo[4,3- ]pyrimidine derivatives 66a and 66b by X-ray diffraction <1994JCX375>. In the solid state, both molecules had the same conformation and were found to be packed in similar manner. The triazole rings proved to be planar, and the conjugation of the Jt-electrons with the lone pair of the N-8 atom was assumed to be the reason of the sofa conformation of the saturated six-membered ring. 

Both the measured dipole moments and H NMR spectra of 1,2-dithiane, its 4,4,5,5-tetradeutero analog, 3,3,6,6-tetramethyl-l,2-dithiane, and the m//ra r-isomers of 3,6-dimethyl-l,2-dithiane provided unequivocal evidence for the chair conformation adopted by the saturated six-membered ring, supported further by X-ray solid-state structures. Other conformers, for example the twist conformer, were not detected . 

Conformational Equilibria in Nitrogen-Containing Saturated Six-Membered Rings... 

Data are available on the 13C spectra of saturated six-membered ring systems (B-79MI20101). The chemical shifts of the ot-, 3- and y-methylene carbon atoms (76JA3778) of compounds of type (41) are summarized in Table 15. 

When two heteroatoms are present in a saturated six-membered ring their effects are approximately additive to within 5 ppm. Observed shifts for a few representative examples are shown in structures (44)-(48). 

If an equatorial hydroxy group on a saturated six-membered ring is changed to axial orientation, the y-carbons with axially oriented hydrogen atoms on the same side of the ring shift to higher field [88, 89, 676, 680]. 

Indolizines with a saturated six-membered ring may be obtained from pyrrole or furan derivatives.225,226 The reaction of 3,4-dimethylpyrryl-magnesium bromide with y-chlorobutyronitrile followed by dilute acid gave 5,6,7,8-tetrahydro-l,2-dimethyl-5-oxoindolizine. 5,6,7,8-Tetrahy-droindolizine can be obtained by the cyclodehydration of 3-(2-furyl)-propylamine over alumina at 400°C. The same product is obtained by the treatment of 3-(2-pyrryl)propyl cyanide with hydrogen chloride in the presence of boron trifluoride etherate catalyst, followed by Wolff-Kishner reduction of the intermediate ketone. 

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