>> If I am converting from scratch and do scan without profile assigned. Then convert adobergb.

Assuming that you work in PS..
When you open an image with no RGB profile assigned and discard the color profile assignment, PS assumes that your image is in your Working RGB profile and then converts from Working RGB into whatever destination profile you did chose.
Adobe RGB is not large enough to cover all the colors that slide film can capture. Chrome Space 100, J Holmes (https://www.josephholmes.com/profiles/about-my-profiles) would be a better choice. ProPhotoRGB is another one but is less "safe".