Table 1. Experimental and DFT Calculations of the Ring Strain Energies (kcal/mol) of Six-Membered Cyclic Phosphoester (1), Five-Membered Cyclic Phosphoester (2), and 3′,5′-Cyclic Phosphoester (3).

method	1	2	(R)-3	(S)-3
experimental		5.9 ± 0.3a
B3LYP/6-31+Ga	0	4.1	6.3	5.9
M06-2X/6-31+Ga	0	5.0	7.2	5.4
B97-D/6-31+Ga	0	4.2	7.0	5.8

^aExperimental value of 2-Me was reported.⁴⁸