27 C.F.R. § 70.145 – Purchasers, holders of security interests, mechanic's lienors, and judgment lien creditors.

(a) Invalidity of lien without notice. The lien imposed by 26 U.S.C. 6321 is not valid against any purchaser (as defined in § 70.143(f) of this part), holder of a security interest (as defined in § 70.143(a) of this part), mechanic's lienor (as defined in § 70.143(b) of this part), or judgment lien creditor (as defined in § 70.143(g) of this part) until a notice of lien is filed in accordance with § 70.148 of this part. Except as provided by 26 U.S.C. 6323, if a person becomes a purchaser, holder of a security interest, mechanic's lienor, or judgment lien creditor after a notice of lien is filed in accordance with § 70.148 of this part, the interest acquired by such person is subject to the lien imposed by 26 U.S.C. 6321.
(b) Cross references. For provisions relating to the protection afforded a security interest arising after tax lien filing, which interest is covered by a commercial transactions financing agreement, real property construction or improvement financing agreement, or an obligatory disbursement agreement, see §§ 70.232, 70.233, and 70.234 of this part, respectively. For provisions relating to the protection afforded to a security interest coming into existence by virtue of disbursements, made before the 46th day after the date of tax lien filing, see § 70.146 of this part. For provisions relating to priority afforded to interest and certain other expenses with respect to a lien or security interest having priority over the lien imposed by 26 U.S.C. 6321, see § 70.147 of this part. For provisions relating to certain other interests arising after tax lien filing, see § 70.231 of this part.
(26 U.S.C. 6323)